Learn the important distinction between "that" vs. "which" on GMAT Sentence Correction.
That vs. which? Restrictive vs. Non-restrictive
A restrictive modifier limits the conceivable scope of a general noun to a relevant subset. For example, consider the difference in the following two sentences:
1) I avoid dogs that appear too excited.
2) I avoid dogs, which appear too excited.
The first sentence conveys that I have a problem some particular category of dogs, those dogs that happen to seem too excited. Conceivably, even someone generally fond of dogs might say this about some narrow subset of dogs. The second sentence is a much more scathing statement: it suggests that I avoid all dogs, and the modifier acts as a judgment, almost a blanket condemnation, that I make about absolutely all dogs. The first, containing the word "that", is a restrictive clause, which narrows the scope from all dogs to a more specific subset. The second, containing the word "which", is a non-restrictive clause, which simply adds a judgment to the whole category of dogs. In a choice between "that" and "which", "that" must be used for the restrictive clause, and "which" for the non-restrictive clause. (BTW, lest any reader fret, outside of the foregoing hypothetical example, I really love all dogs!)
How do I pick?
How can you tell whether a clause should be restrictive or not? Well, one easy trick is to state the sentence without the clause. A restrictive clause provides vital information, so when it is dropped, the meaning of the sentence changes. A non-restrictive clause may provide helpful or interesting information, but when it is dropped, there is absolutely no ambiguity about the identity of the noun, and the sentence retains the same meaning. In the following sentences, see if you can tell whether the underlined clause should be restrictive or non-restrictive, and thus, whether the word "that" or "which" should be used.
1) Mortimer can't drive cars (that/which) have a standard transmission.
2) My favorite bridge is the Brooklyn Bridge (that/which) spans the East River in New York City.
3) Dr. Martin Luther King openly questioned laws (that/which) instituted injustice.
In sentence #1, it is certainly not true that all cars have a standard transmission! The vast majority of modern cars have automatic transmission, which means the only logical choice is the restrictive clause. Dropping the clause seems to suggest that Mortimer can't drive at all, which has a different meaning than a statement about his ability to drive a car with standard transmission. In #4, there is only one Brooklyn Bridge in the whole world, so dropping the clause leaves absolutely no doubt about its identity: we need the non-restrictive clause. In #3, Dr. King was not a wild anarchist who questioned absolutely all laws; quite to the contrary, he was highly principled individual who raised profound moral questions about a particular subset of laws, those which embodied some form of injustice. Therefore, the restrictive clause is needed. The corrected sentences are:
1) Mortimer can't drive cars that have a standard transmission.
2) My favorite bridge is the Brooklyn Bridge, which spans the East River in New York City
3) Dr. Martin Luther King openly questioned laws that instituted injustice.
What about commas?
You may have noticed another subtle difference. A non-restrictive clause needs to be set off from the noun it modifies by commas. Commas should not set off a restrictive clause from the noun it modifies. When there's a "that"/"which" distinction, that can tells whether the clause is restrictive or not. When the same relative pronoun is used in both cases, sometimes our only clue is the punctuation. Consider these two sentences.
1) Bartholomew doesn't like people who talk too much.
2) Bartholomew doesn't like people, who talk too much.
Just one comma, but a world of difference! The first, without commas, is a restrictive clause which narrows the topic down from all human beings to some subset, just those folks who (according to Bartholomew) talk too much. We would need to know a little more about our hypothetical friend Bartholomew to know how reasonably his judgment is, but at least in some interpretations, we easily could imagine this is a sentiment that many people might hold. In the second sentence, the clause is separate by a comma, which tells is it is non-restrictive: in that sentence, our friend Bartholomew seems to have some major issues: he doesn't like people in general, the whole human race at large, and the modifier implies that he doesn't like people because all people, the whole lot of them, talk too much, at least according to him. It would be exceeding hard to imagine such a person has a sane and happy life! This exemplifies the profound difference commas can make in the meaning of a sentence.
If you understand this distinction between restrictive vs. non-restrictive clauses, you will master one of the most confusing areas of GMAT Sentence Correction --- and you will get "that" vs. "which" right every time!
This post was written by Mike McGarry, GMAT Expert at Magoosh, and originally posted here.