A common point of confusion among those studying for the GMAT—as well as the general writing public—is the difference between “that” and “which.”  Knowing which of these words to use in a given situation can often allow you to eliminate a couple of answer choices, speeding up your verbal test-taking and ultimately helping to improve your score.

The simplest way to remember the difference is by noting that “which” introduces information that isn’t crucial to the meaning of the sentence, known as a non-restrictive or non-essential clause, while “that” introduces a restrictive clause.  For example,

I am going to wear the sweater that is blue.

I am going to wear the sweater, which is blue.

In the former sentence, “that is blue” functions to restrict the meaning; the implication is that there is more than one sweater, but only one blue one, and the blue one is the one I’m going to wear.  In the latter sentence, the phrase “which is blue” merely provides more information about the sweater.  The sentence’s meaning wouldn’t change dramatically without the “which is blue,” because the information is non-essential.

An example of this kind of sentence on GMAT Sentence Corrections might look like this:

Thanks to medical research, many diseases which might have been fatal at one time are now easily prevented by childhood vaccines.

A.  which might have been fatal at one time are now easily prevented by childhood vaccines

B.  that might have been fatal at one time are now easily prevented for childhood vaccines

C.  which might at one time have been fatal without childhood vaccines

D.  that might have been fatal at one time are now easily prevented by childhood vaccines

E.  which might, without the current prevention by childhood vaccines, have been fatal

In this sentence, the meaning should be restricted; the diseases to which the sentence refers are specifically those that might have been fatal at one time.  Therefore, the correct answer choice will use “that” instead of “which.”  Choice A can therefore be eliminated, as can C and E.  In addition, the structures of both C and E are sentence fragments; everything following “which” in those choices functions to describe “diseases,” leaving the sentence without a central verb.  Choices B and D both use “that,” but choice B uses the wrong idiom: it says “prevented for” instead of “prevented by,” which changes the meaning of the sentence.

A second example might look like this:

Global warming that has attracted a great deal of media coverage recently will be a central issue in the upcoming campaign, since both candidates champion environmental causes.

A.  Global warming that has attracted a great deal of media coverage recently will be

B.  Global warming that will be attracting a great deal of media coverage recently will be

C.  Global warming, which will be attracting a great deal of media coverage recently, will be

D.  Global warming, which recently is being the subject of a great deal of media coverage will be

E.  Global warming, which has attracted a great deal of media coverage recently, will be

Here, the adjective clause “has attracted a great deal of media coverage recently” should be non-restrictive.  There is no indication that a specific kind of global warming will be a central issue.  Instead, the sentence implies that global warming will be a central issue in the upcoming campaign, and “global warming” is further described as having “attracted a great deal of media coverage recently”.  The meaning of the sentence would not be substantially changed by removing the information about media coverage.  Therefore, “which” is appropriate in this sentence.  A and B can be eliminated for this reason; in addition, B can be eliminated because the future tense “will be attracting” is inconsistent with “recently.”  Choice C has the same verb tense issue as choice B, and can therefore be eliminated.  Choice D uses “being,” which is almost never correct on the GMAT, and also neglects to put the necessary comma after “coverage” at the end of the adjective clause.  The final choice, E, does everything right: the verb tenses are appropriate, punctuation is correct, and “which” is properly used.

FINAL THOUGHT: Separating restrictive and non-restrictive or non-essential clauses allows you to correctly choose whether to use “that” or “which” in GMAT Sentence Corrections, and like most grammar issues, it becomes much easier with practice on Grockit!