This is a common and seemingly easily resolvable grammatical conundrum — when does one use which and when does one use that? Most test-takers who have prepared for GMAT® Sentence Correction will have this answer at the tip of their tongues — essential/restrictive and non-essential/non-restrictive clause.

What they mean is that which is used to state information that is not essential while that is used to state essential information.

A easier way to remember this is by looking at the pair of sentences below:
1. These are the keys to the fourth car in the parking row, which is black.
2. These are the keys to the fourth car in the parking row that is black.

From the first sentence you would get the keys to the fourth car in the parking row; the sentence gives you some additional information, namely that it is black in colour; even without this information you could have known which car you have the keys to — the fourth car in the row.

From the second sentence you would get the keys to the fourth black car in the parking row and not the fourth car!

So it can be seen that which is preceded by a comma and usually gives non-essential or parenthetical information (information that can be put in brackets) whereas that is used to give essential information.

While GMAT® Sentence Correction questions rarely test this rule, an understanding of this concept can be used to crash your solving time on some questions. For example let’s look at the GMAT® Sentence Correction question below.

Intended primarily to stimulate family summer travel, the new airfare, which allows both an adult and a child to fly for the price of one ticket, and also shortens the advance-purchase requirement for family travel to a minimum of seven days rather than fourteen.

(A) and also shortens the advance-purchase requirement for family travel to a minimum of seven days rather than
(B) and also lessens the advance-purchase requirement for family travel to a seven-day minimum from
(C) also shortens the advance-purchase requirement for family travel to a minimum of seven days rather than that of
(D) also lessens the advance-purchase requirement for family travel to a seven-day minimum from
(E) also shortens the advance-purchase requirement for family travel to a minimum of seven days rather than

As discussed earlier, which is used to provide non-essential or additional information. This means that reading the sentence without reading the information given after which will not hamper your understanding of the sentence.

So when there is information in the non-underlined part preceded by which it can ignored. The above sentence for example can be read by ignoring the non-essential part – , which allows both an adult and a child to fly for the price of one ticket, — the sentence thus becomes:

Intended primarily to stimulate family summer travel, the new airfare and also shortens the advance-purchase requirement for family travel to a minimum of seven days rather than fourteen.

From this is it very obvious that there is no need for the and after airfare, hence options (A) and (B) are eliminated.

Between the last three options (D) is eliminated as lessens is always used for uncountable quantities and not countable quantities like number of days (lessens the pain, for example). Between (C) and (E), the former incorrectly adds a that of at the end which is not required. Hence, (E).

GMAT SC questions are deliberately designed with distractors, one of them being parenthetical information using a comma and a which.

One classic trap set in this way is the structure given below:

Subject is introduced (singular), which…………………………..some information involving a plural…………., verb (plural)…….

A spatial and informational distance is introduced between the subject and the verb or some other two elements (like in the question discussed above) and then the error is introduced. The gap distance tricks test-takers into not spotting the error, resulting in test-takers trying to look for other errors.

When you review such a question and realize you have made an error, you tend to think of it as a silly mistake — it was only subject-verb, something I knew. You tend to think of the error, to use a tennis term, as an unforced error, whereas in reality you have stepped into a deliberately laid trap.

Preparing for the GMAT, especially for SC is a lot about reviewing the answer explanations well and trying to find out what made you commit the mistake. Given the highly standardized nature of the test you find a pattern to the traps, you will realize that the reason for your error is more than just silliness on your part.

In the next SC post we will look at more examples that cover this kind of trap.