In its place, though, revising for correctness matters deeply, because your prose may be unreadable without it. If your prose is ungrammatical, you will not only risk incoherence (in which case your readers will not be able to follow what you are saying), but you will also inadvertently invite them to dismiss you as illiterate. Is it fair of readers to reject your ideas because of the way you've phrased them? Perhaps not, but the fact is they often will. A great many readers regard technical errors as an inattention to detail that also signals sloppiness at more important levels of thinking. In producing writing that contains such errors, you risk not only distracting readers from your message but also undermining your authority to deliver the message in the first place.
 

You get a paper back, and it's a sea of red ink. But if you look more closely, you'll often find that you haven't made a million mistakes-you've made only a few, but over and over in various forms. This phenomenon is what the rhetorician Mina Shaughnessy addressed in creating the category of Basic Writing Errors, or BWEs. Shaughnessy argues that in order to improve your writing for correctness, you need to do two things:

-- Look for a pattern of error, which will require you to understand your own logic in the mistakes you typically make.

-- Recognize that not all errors are created equal, which means that you need to address errors in some order of importance-beginning with those most likely to interfere with your readers' understanding.

Our BWE guide does not cover all of the rules of grammar, punctuation, diction, and usage, such as where to place the comma or period when you close a quotation or whether or not to write out numerals. For comprehensive coverage of the conventions of standard written English, you can consult one of the many handbooks available for this purpose. Our purpose is to provide a short guide to grammar-identifying the most common errors, providing remedies, and offering the logic that underlies them. This chapter's coverage of nine basic writing errors and how to fix them will help you eliminate most of the problems that routinely turn up in student essays. We have arranged the error types in a hierarchy, moving in descending order of severity (from most to least problematic).

Most of the relevant terminology for talking about grammar appears within the discussion of the Basic Writing Errors. For definitions of terms that for reasons of space are not included there, we have compiled a "Glossary of Grammatical Terms," which appears at the end of the chapter. The key explanatory terms used in the BWE list have been boldfaced to indicate that they can also be found, alphabetically organized, in this glossary. Some of the nine BWEs also recur briefly in the glossary, but we have tried to avoid unnecessary repetition.
 

BWE 1: SENTENCE FRAGMENTS

The most basic of writing errors, a sentence fragment, is a group of words punctuated like a complete sentence but lacking the necessary structure: it is only part of a sentence. Typically, a sentence fragment occurs when the group of words in question (a) lacks a subject, (b) lacks a predicate, or (c) is a subordinate (or dependent) clause.

To fix a sentence fragment, either turn it into an independent clause by providing whatever is missing--a subject or a predicate-or attach it to an independent clause upon which it can depend.
 

Noun Clause (No Predicate) as a Fragment

A world where imagination takes over and sorrow is left behind.

The fragment is not a sentence but a noun clause--a sentence subject with no predicate. The fragment lacks a verb that would assert something about the subject. (The verbs takes over and is left are in a dependent clause created by the subordinating conjunction where.)

Corrections

A world arose where imagination takes over and sorrow is left behind.
[new verb matched to A world]
She entered a world where imagination takes over and sorrow is left behind.
[new subject and verb added]

The first correction adds a new verb (arose). The second introduces a new subject and verb, converting the fragment into the direct object of She entered.
 

Verbal as a Fragment

Falling into debt for the fourth consecutive year.

Falling in the fragment above is not a verb. Depending on the correction, Falling is either a verbal or part of a verb phrase.

Corrections

The company was falling into debt for the fourth consecutive year.
[subject and helping verb added]
Falling into debt for the fourth consecutive year led the company to consider relocating.
[new predicate added]
Falling into debt for the fourth consecutive year, the company considered relocating.
[new subject and verb added]

In the first correction, the addition of a subject and the helping verb was converts the fragment into a sentence. The second correction turns the fragment into a gerund phrase functioning as the subject of a new sentence. The third correction converts the fragment into a participial phrase attached to a new independent clause.

Subordinate Clause as a Fragment

I had an appointment for 11:00 and was still waiting at 11:30. Although I did get to see the dean before lunch.

Although is a subordinating conjunction that calls for some kind of completion. Like if, when, because, whereas, and other subordinating conjunctions (see glossary), although always makes the clause that it introduces dependent.

One way to correct a fragment is to replace the period with a dash: "The campaign required commitment, Not just money." becomes "The campaign required commitment--not just money." The dash offers you one way of attaching a phrase or clause to a sentence without having to construct another independent clause. In short, it's succinct. (Compare the correction that uses the dash with another possible correction: "The campaign required commitment. It also needed money.") Moreover, with the air of sudden interruption that the dash conveys, it can capture the informality and immediacy that the intentional fragment offers a writer.

You should be wary of overusing the dash in this way, as the slightly more presentable cousin of the intentional fragment. The energy it carries can clash with the decorum of formal writing contexts; its staccato effect quickly becomes too much of a good thing.

One alternative to this usage of the dash, in some cases, is the colon. It can substitute because it also can be followed by a phrase, a list, or a clause. As with the dash, it must be preceded by an independent clause, And it, too, carries dramatic force because it abruptly halts the flow of the sentence. The colon, however, does not convey informality In place of a slapdash effect, it offers a spotlight on what is to follow it. Hence, as in this sentence, it is especially appropriate for setting up certain kinds of information: explanations, lists, or results. In the case of results, the cause or action precedes the colon; the effect or reaction follows it.

Before leaving this hint box, let us quickly offer the other legitimate use of the dash: to enclose information within a sentence. In this use, dashes precede and follow the information, taking the role usually assigned to commas. Consider the following example:
Shortly before the election-timing its disclosures for maximal destructive effectthe candidate's campaign staff levied a series of charges against the incumbent.

Note that if the information within the dashes is omitted, the sentence must still read grammatically. That is the rule for using dashes in this way.

Correction

I had an appointment for 11:00 and was still waiting at 11:30, although I did get to see the dean before lunch. [fragment attached to preceding sentence]

As the correction demonstrates, the remedy lies in attaching the fragment to an independent clause on which it can depend (or alternatively, making the fragment into a sentence by dropping the conjunction).

Sometimes writers use sentence fragments deliberately, usually for rhythm and emphasis or to create a conversational tone. In less formal contexts, they are generally permissible, but you run the risk that the fragment will not be perceived as intentional. In formal writing assignments, it is safer to avoid intentional fragments.
 

TEST YOURSELF:

FRAGMENTS There are fragments in each of the three examples below, probably the result of their proximity to legitimate sentences. What's the problem in each case, and how would you fix it?

Margaret Mead studied non-Western cultures in such works as Coming of Age in Samoa. And influenced theories of childhood development in America.

The catastrophe resulted from an engineering flaw. Because the bridge lacked sufficient support.

In the 1840s the potato famine decimated Ireland. It being a country with poor soil and antiquated methods of agriculture.
 
 

A comma splice consists of two independent clauses connected ("spliced") with a comma; a fused or run-on sentence combines two such clauses with no conjunction or punctuation. The remedies for both comma splices and fused sentences are the same:

1. Place a conjunction (such as and or because) between the clauses.
2. Place a semicolon between the clauses.
3. Make the clauses into separate sentences.

All of these solutions solve the same logical problem: they clarify the boundaries of the independent clauses for your readers.
 

Comma Splice

He disliked discipline, he avoided anything demanding.

Correction

Because he disliked discipline, he avoided anything demanding. [subordinating conjunction added]

Comma Splice

Today most TV programs are violent, almost every program is about cops and detectives.

Correction

Today most TV programs are violent; almost every program is about cops and detectives. [semicolon replaces comma]
 

Since the two independent clauses in the first example contain ideas that are closely connected logically, the most effective of the three comma-splice remedies is to add a subordinating conjunction (Because) to the first of the two clauses, making it depend on the second. For the same reason-close conceptual connection--the best remedy for the next comma splice is to substitute a semicolon for the comma. The semicolon signals that the two independent clauses are closely linked in meaning. In general, you can use a semicolon where you could also use a period.

The best cures for the perpetual comma splicer are to learn to recognize the difference between independent and dependent clauses and to get rid of the "pause theory" of punctuation. All of the clauses in our two examples are independent. As written, each of these should be punctuated not with a comma but a period or a semicolon. Instead, the perpetual comma splicer usually acts on the pause theory: because the ideas in the independent clauses are closely connected, the writer hesitates to separate them with a period. And so the writer inserts what he or she takes to be a shorter pause-the comma. But a comma is not a breath mark; it provides readers with specific grammatical information, in this case (erroneously) that there is only one independent clause separated by the comma from modifying information. In the corrections, by contrast, the semicolon sends the appropriate signal to the reader: the message that it is joining two associated but independent statements. (A coordinating conjunction such as and would also be grammatically correct, though possibly awkward.)
 

Fused Sentence

The Indo-European language family includes many groups most languages in Europe belong to it.

Correction

The Indo-European language family includes many groups. Most languages in Europe belong to it.
[period inserted after first independent clause]
 

You could also fix this fused sentence with the coordinating conjunction and. Alternatively, you might condense the whole into a single independent clause:

Most languages in Europe belong to the Indo-European language family.
 

Comma Splices with Conjunctive Adverbs

Quantitative methods of data collection show broad trends, however, they ignore specific cases.
Sociobiology poses a threat to traditional ethics, for example, it asserts that human behavior is genetically motivated by the "selfish gene" to perpetuate itself.

Corrections

Quantitative methods of data collection show broad trends; however, they ignore specific cases. [semicolon replaces comma before however]
Sociobiology poses a threat to traditional ethics; for example, it asserts that human behavior is genetically motivated by the "selfish gene" to perpetuate itself. [semicolon replaces comma before for example]

Both of these examples contain one of the most common forms of comma splices. Both of them are compound sentences-that is, they contain two independent clauses. (See Chapter 9, the section "The Compound Sentence.") Normally, connecting the clauses with a comma and a conjunction would be correct: for example, "Most hawks hunt alone, but osprey hunt in pairs." In the two comma splices given, however, the independent clauses are joined by transitional expressions known as conjunctive adverbs.  When conjunctive adverbs are used to link two independent clauses, they always require a semicolon. By contrast, when a coordinating conjunction links the two clauses of a compound sentence, it is always preceded by a comma.

In most cases, depending on the sense of the sentence, the semicolon precedes the conjunctive adverb and has the effect of clarifying the division between the two clauses. There are exceptions to this general rule, though, as in the following sentence.

The lazy boy did finally read a book, however; it was the least he could do.

Here however is a part of the first independent clause, qualifying its claim. The sentence thus suggests that the boy was not totally lazy, since he did get around to reading a book. Note how the meaning changes when however becomes the introductory word for the second independent clause:

The lazy boy did finally read a book; however, it was the least he could do.

Here the restricting force of however suggests that reading the book was not much of an accomplishment.
 

TEST YOURSELF:

COMMA SPLICES

What makes each of the sentences below a comma splice? What would be the best way to fix each one and why?

"Virtual reality" is a new buzzword, so is "hyperspace."

Many popular cures for cancer have been discredited, nevertheless, many people continue to buy them.

Elvis Presley's home, Graceland, attracts many musicians as a kind of shrine, even Paul Simon has been there.

She didn't play well with others, she sat on the bench and watched.

BWE 3: ERRORS IN SUBJECT-VERB AGREEMENT

The subject and the verb must agree in number, singular subjects taking singular verbs and plural subjects taking plural verbs. Errors in subject-verb agreement usually occur when a writer misidentifies the subject or verb of a clause.
 

Agreement Problem

Various kinds of vandalism has been rapidly increasing.

Correction

Various kinds of vandalism have been rapidly increasing.
[verb made plural to match kinds]

Hints: Standard Written English

The term standard written English refers to language that conforms to the rules and conventions adhered to by the majority of English-speaking writers. The fact is, however, that not all speakers of English grow up hearing, reading, and writing standard written English. Some linguistic cultures in America, for example, follow a different set of conventions for subject/verb agreement. Their speakers do not differentiate singular from plural verb forms with a terminal -s, as in standard English:

She walks home after work.
They walk home after work.

Some speakers of English do not observe this distinction, so that the first sentence above becomes:

She walk home after work.

These two ways of handling subject/verb agreement are recognized by linguists not in terms of right versus wrong, but in terms of dialect difference. A dialect is a variety of a language that is characteristic of a region or culture and is sometimes unintelligible to outsiders. The problem for speakers of a dialect that differs from the norm is that they can't always rely on their ear-on what sounds right-when they are editing according to the rules of standard written English. If you consistently fail to detect a particular kind of error because it looks and sounds right to you, the best solution is to add a proofreading stage to your final draft in which you check every sentence only for that error.

When you isolate the grammatical subject (kinds) and the verb (has) of the original sentence, you can tell that they do not agree. While vandalism might seem to be the subject because it is closest to the verb, it is actually the object of the preposition of The majority of agreement problems arise from mistaking the object of a preposition for the actual subject of a sentence. If you habitually make this mistake, you can begin to remedy it by familiarizing yourself with the most common prepositions.
 

Agreement Problem

Another aspect of territoriality that differentiates humans from animals are their possession of ideas and objects.

Correction

Another aspect of territoriality that differentiates humans from animals is their possession of ideas and objects. [verb made singular to match subject aspect]

The subject of the sentence is aspect. The two plural nouns (humans and animals) probably encourage the mistake of using a plural verb (are), but humans is part of the that clause modifying aspect, and animals is the object of the preposition from.
 

Agreement Problem

The Republican and the Democrat both believe in doing what's best for America, but each believe that the other doesn't understand what's best.

Correction

The Republican and the Democrat both believe in doing what's best for America, but each believes that the other doesn't understand what's best. [verb made singular to agree with subject each]

The word each is always singular, so the verb must be singular as well (believes). The presence of a plural subject and verb in the sentence's first independent clause (The Republican and the Democrat both believe) has probably encouraged the error.
 
 

TEST YOURSELF:

SUBJECT-VERB AGREEMENT

Test yourself by diagnosing and correcting the error in the example below:

The controversies surrounding the placement of Arthur Ashe's statue in

Richmond was difficult for the various factions to resolve.
 
 

This error involves an illogical mismatch between subject and predicate. If you continually run afoul of faulty predication, you might use the exercises in a handbook to drill you on isolating the grammatical subjects and verbs of sentences, since that is the first move you need to make in fixing the problem.

Shift

In 1987, the release of more information became available.

Correction

In 1987, more information became available for release.
[new subject]

It was the information, not the release, that became more available. The correction relocates information from its position as object of the preposition of to the subject position in the sentence; it also moves release into a prepositional phrase.

Shift

The busing controversy was intended to rectify the inequality of educational opportunities.

Correction

Busing was intended to rectify the inequality of educational opportunities.
[new subject formulated to match verb]

The controversy wasn't intended to rectify, but busing was.
 

TEST YOURSELF: FAULTY PREDICATION

Identify and correct the faulty predication in the example below:

The subject of learning disabilities is difficult to identify accurately.
 

BWE 5: ERRORS IN PRONOUN REFERENCE

There are at least three forms of this problem. All of them involve a lack of clarity about who or what a pronoun (a word that substitutes for a noun) refers to. The surest way to avoid difficulties is to make certain that the pronoun relates back unambiguously to a specific word, known as the antecedent. In the sentence "Nowadays appliances don't last as long as they once did," the noun appliances is the antecedent of the pronoun they.

Pronoun-Antecedent Agreement

A pronoun must agree in number (and gender) with the noun or noun phrase that it refers to.

Pronoun Error

It can be dangerous if a child, after watching television, decides to practice what they saw.

Corrections

It can be dangerous if children, after watching television, decide to practice what they saw. (antecedent (and verb) made plural to agree with pronoun]
It can be dangerous if a child, after watching television, decides to practice what he or she saw. [singular pronouns substituted to match singular antecedent child]

The error occurs because child is singular but its matching pronoun, they, is plural. The first correction makes both plural; the second makes both singular. You might also observe in the first word of the example-the impersonal it-an exception to the rule that pronouns must have antecedents.
 

TEST YOURSELF: PRONOUN-ANTECEDENT AGREEMENT

What is wrong with the following sentence, and how would you fix it?

Every dog has its day, but all too often when that day happens, they can be found barking up the wrong tree.

Ambiguous Reference

A pronoun should have only one possible antecedent.

Pronoun Error

Children like comedians because they have a sense of humor.

Corrections

Because children have a sense of humor, they like comedians. [subordinate because clause placed first, and relationship between noun children and pronoun they tightened]

Children like comedians because comedians have a sense of humor
[pronoun eliminated and replaced by repetition of noun]
 

Hints: Sexism and Pronoun Usage
 

Errors in pronoun reference sometimes occur because of a writer's praiseworthy desire to avoid sexism. In most circles, the following correction of the preceding example would be considered sexist:
It can be dangerous if a child, after watching TV, decides to practice what he saw.

Though the writer of such a Sentence may intend he to function as a genderneutral, impersonal pronoun, it in fact excludes girls on the basis of gender, Implicitly, it also conveys sexual stereotypes (for example, that only boys are violent, or perhaps stupid enough to confuse TV with reality).

The easiest way to avoid the problem of sexism in pronoun usage usually lies in putting things into the plural, since plural pronouns (we, you, they) have no gender. (See the use of children in the first correction of the "Pronoun-Antecedent Agreement" example.) Alternatively, you can use the phrase he or she, as in the second correction above. Many readers, however, find lie or she and its variant, s/he to be awkward constructions. Another remedy lies in rewriting the sentence to avoid pronouns altogether, as in the following revision:
It can be dangerous if a child, after watching television, decides to practice some violent activity portrayed on the screen.

Does they in the original example refer to children or comedians? The rule in such cases of ambiguity is that the pronoun refers to the nearest possible antecedent, so here comedians possess the sense of humor, regardless of what the writer may intend. As the corrections demonstrate, either reordering the sentence or repeating the noun can remove the ambiguity.

TEST YOURSELF: AMBIGUOUS REFERENCE

As you proofread, it's a good idea to target your pronouns to make sure that they cannot conceivably refer to more than one noun. What's wrong with the following sentences, and how would you fix them?

Alexander the Great's father, Philip of Macedon, died when he was twentysix.

The committee could not look into the problem because it was too involved.

Broad Reference

Broad reference occurs when a pronoun refers loosely to a number of ideas expressed in preceding clauses or sentences. It causes confusion because the reader cannot be sure which of the ideas the pronoun refers to.

Pronoun Error

As a number of scholars have noted, Sigmund Freud and Karl Marx offered competing but also at times complementary critiques of the dehumanizing tendencies of Western capitalist society. We see this in Christopher Lasch's analysis of conspicuous consumption in The Culture of Narcissism.

Correction

As a number of scholars have noted, Sigmund Freud and Karl Marx offered competing but also at times complementary critiques of the dehumanizing tendencies of Western capitalist society. We see this complementary view in Christopher Lasch's analysis of conspicuous consumption in The Culture of Narcissism. [broad this clarified by addition of noun phrase]

The word this in the second sentence of the uncorrected example could refer to the fact that a number of scholars have noted the relationship between Marx and Freud, to the competition between Freud's and Marx's critiques of capitalism, or to the complementary nature of their critiques.

Broad reference most commonly occurs when this is used as a pronoun; the remedy is generally to avoid using the word as a pronoun. Instead, convert this into an adjective, and let it modify some noun that more clearly specifies the referent: this complementary view, as in the correction or, alternatively, this competition or this scholarly perspective.
 

TEST YOURSELF: BROAD REFERENCE Locate the errors in the following examples and provide a remedy:

Regardless of whether the film is foreign or domestic, they can be found in your neighborhood video store.

Many experts now claim that dogs and other higher mammals dream; for those who don't own such pets, this is often difficult to believe.
 
 

Before you can detect misplaced modifiers, you first need to understand what modifiers are. Modifiers are words or groups of words used to qualify, limit, intensify, or explain some other element in a sentence. A misplaced modifier is a word or phrase that appears to modify the wrong word or words.
 

Misplaced Modifier

At the age of three he caught a fish with a broken arm,

Correction

At the age of three the boy with a broken arm caught a fish.
[noun replaces pronoun; prepositional phrase revised and relocated]

The original sentence mistakenly implies that the fish has a broken arm.
 

Misplaced Modifier

According to legend, General George Washington crossed the Delaware and celebrated Christmas in a small boat.

Correction

According to legend, General George Washington crossed the Delaware in a small boat and then celebrated Christmas on shore.
[prepositional phrase relocated; modifiers added to second verb]
 

As a general rule, you can avoid misplacing a modifier by keeping it as close as possible to what it modifies. Thus, the second correction removes the implication that Washington celebrated Christmas in a small boat. When you cannot relocate the modifier, separate it from the rest of the sentence with a comma to prevent readers from connecting it to the nearest noun.
 

A dangling participle creates a particular kind of problem in modification: the noun or pronoun that the writer intends the participial phrase to modify is not actually present in the sentence.

Dangling Participle

After debating the issue of tax credits for the elderly, the bill passed in a close vote.

Correction

After debating the issue of tax credits for the elderly, the Senate passed the bill in a close vote.
[appropriate noun added for participle to modify]

The bill did not debate the issue, as the original example implies. As the correction demonstrates, fixing a dangling participle involves tightening the link between the activity implied by the participle (debating) and the entity performing that action (the Senate).
 

TEST YOURSELF: MODIFICATION ERRORS

Find the modification errors in the examples below. How would you correct each of them?

After eating their sandwiches, the steamboat left the dock.

The social workers saw an elderly woman on a bus with a cane standing up.

Crossing the street, a car hit the pedestrian.
 
 

Adding 's to most singular nouns will make them show possession: the plant's roots, the accountant's ledger. You can add the apostrophe alone, without the s, to make plural nouns that already end with s show possession: the flowers' fragrances, the ships' berths.

Apostrophe Error

The loyal opposition scorned the committees decisions.

Corrections

The loyal opposition scorned the committee's decisions.
The loyal opposition scorned the committees' decisions.
[possessive apostrophe added]

The first correction assumes there was one committee; the second assumes there were two or more.

Apostrophe Error

The advisory board swiftly transacted it's business.

Correction

The advisory board swiftly transacted its business.
[apostrophe dropped]

Unlike possessive nouns, possessive pronouns (my, your, yours, her, hers, his, its, our, ours, their, theirs) do not take an apostrophe. (See BWE 9.)
 

TEST YOURSELF: POSSESSIVE APOSTROPHES

Find the error in the following sentence.

The womens movement has been misunderstood by many of its detractors.

BWE 8: COMMA ERRORS

As with other rules of punctuation and grammar, the many that pertain to comma usage share an underlying aim: to clarify the relationships among the parts of a sentence. Commas separate the parts of a sentence. They are particularly useful in separating the main clause from dependent elements, such as subordinate clauses and long prepositional phrases. One of their primary uses, then, is to help your readers distinguish the main clause from the rest of the sentence.
 

Comma Error

After eating the couple went home.

Correction

After eating, the couple went home.
[comma added before independent clause]
 

The comma after eating is needed to keep the main clause visible or separate; it marks the point at which the prepositional phrase ends and the independent clause begins. Without this separation, readers would be invited to contemplate cannibalism as they move across the sentence.
 

Comma Error

In the absence of rhetoric study teachers and students lack a vocabulary for talk-
ing about their prose.

Correction

In the absence of rhetoric study, teachers and students lack a vocabulary for talking about their prose.
[comma added to separate prepositional phrase from main clause]
 

Readers have to read the sentence twice to find out where the prepositional phrase ends-with study---in order to figure out where the main clause begins.
 

Comma Error

Dog owners, despite their many objections will have to obey the new law.

Correction

Dog owners, despite their many objections, will have to obey the new law.
[single comma converted to a pair of commas]
 

A comma is needed after objections in order to isolate the phrase in the middle of the sentence (despite their many objections) from the main clause. This phrase is nonrestrictive; that is, the information it contains provides additional information that is not essential to the meaning of what it modifies. The test of nonrestrictive phrases and clauses is to see if they can be omitted without substantially changing the message that a sentence conveys ("Dog owners will have to obey the new law"). Nonrestrictive elements always take two commas--a comma 11 sandwich"--to set them off. Using only one comma separates the sentence's subject (dog owners) from its predicate (will have to obey) illogically. This problem is easier to see in a shorter sentence. You wouldn't, for example, write "I fell down."
 

Comma Error

Most people regardless of age like to spend money.

Correction

Most people, regardless of age, like to spend money.
[comma sandwich added]
 

Commas enclose the nonrestrictive elements in the corrected example above. You could omit this information without significantly affecting the sense. Such is not the case in the two examples below.
 

Comma Error

People, who live in glass houses, should not throw stones.

Correction

People who live in glass houses should not throw stones.
[commas omitted]

Comma Error

Please return the library book, that I left on the table.

Correction

Please return the library book that I left on the table.
[comma omitted]
 

It is incorrect to place commas around who live in glass houses or a comma before that I left on the table. Each of these is a restrictive clause-that is, it contains information that is an essential part of what it modifies. In the first sentence, for example, if who live in glass houses is left out, the meaning of the sentence is lost: People should not throw stones. Who is defined by restricting it to people in the category of glass-house dwellers. Similarly, in the second example the that clause contributes an essential meaning to book-the sentence is referring to not just any book but a particular one, the one on the table.

As a general rule, if the information in a phrase or clause can be omitted--if it is nonessential and therefore nonrestrictive it needs to be separated by commas from the rest of the sentence. Also be aware that restrictive clauses use that and nonrestrictive clauses use which.
 

TEST YOURSELF: COMMA ERRORS

How would you fix the following example?

The book which I had read a few years ago contained a lot of outdated data.

BWE 9: SPELLING & DICTION ERRORS THAT INTERFERE WITH MEANING

Misspellings are always a problem in a final draft, insofar as they undermine your authority by inviting readers to perceive you as careless (at best). If you make a habit of using the spell-checking function of a word processor, you will take care of most misspellings. But the problems that a spell-checker won't catch are the ones that can often hurt you most. These are actually diction errors-incorrect word choices in which you have confused one word with another that it closely resembles. In such cases, you have spelled the word correctly, but it's the wrong word. Since it means something other than what you've intended, you end up misleading your readers.

The best way to avoid this problem is to memorize the differences between pairs of words that are commonly confused with each other but have distinct meanings. The following examples illustrate a few of the most common and serious of these errors. Most handbooks contain a glossary of usage that cites more of these sites of confusion.
 

Spelling /Diction Error: It's versus Its

Although you can't tell a book by it's cover, its fairly easy to get the general idea from the introduction.

Correction

Although you can't tell a book by its cover, it's fairly easy to get the general idea from the introduction. [apostrophe dropped from possessive and added to contraction]
 

It's is a contraction for it is. Its is a possessive pronoun meaning "belonging to it." If you confuse the two, it's likely that your sentence will, at least initially, mislead its readers. (See BWE 7.)
 

Spelling/Diction Error: Their versus There versus They're

Their are ways of learning about the cuisine of northern India besides going their to watch the master chefs and learn there secrets-assuming their willing to share them.

Correction

There are ways of learning about the cuisine of northern India besides going there to watch the master chefs and learn their secretsassuming they're willing to share them.
[expletive there, adverb there, possessive pronoun their, and contraction they're inserted appropriately]

There as an adverb normally refers to a place; there can also be used as an expletive to introduce a clause, as in the first usage of the correction. (See the discussion of expletives in chapter 9, the section "Cutting the Fat.") Their is a possessive pronoun meaning "belonging to them." They're is a contraction for they are.
 

Spelling /Diction Error: Then versus Than

If a person would rather break a law then obey it, than he or she must be willing to face the consequences.

Correction

If a person would rather break a law than obey it, then he or she must be willing to face the consequences.
[comparative than distinguished from temporal then]

Than is a conjunction used with comparisons: rather x than y. Then is an adverb used to indicate what comes next in relation to time: first x, then y.
 

Spelling/Diction Error: Effect versus Affect

It is simply the case that BWEs adversely effect the way that readers judge what a writer has to say. It follows that writers who include lots of BWEs in their prose may not have calculated the disastrous affects of these mistakes.

Correction

It is simply the case that BWEs adversely affect the way that readers judge what a writer has to say. It follows that writers who include lots of BWEs in their prose may not have calculated the disastrous effects of these mistakes.
[verb affect and noun effects inserted appropriately]

In their most common usages, affect is a verb meaning "to influence" and effect is a noun meaning "the result of an action or cause." The confusion of affect and effect is enlarged by the fact that both of these words have secondary meanings: the verb to effect means "to cause or bring about"; the noun affect is used in psychology to mean "emotion or feeling." Thus, if you confuse these two words, you will inadvertently make a meaning radically different from the one you intend.