'That' and 'Which'

On GMAT Sentence Correction questions, you will frequently be asked to chose between 'that' and 'which'. For knowledge's sake remember that both of these belong to a small group of words known as relative pronouns, so called because they are used to relate parts of a sentence to one another.

Consider these 2 sentences:

1. The fifth car, which is black in color, belongs to Jack.
   
   
2. The fifth car that is black in color belongs to Jack.

Are both of these sentences referring to the same car? Not necessarily.

The first sentence is definitely referring to the fifth car and we are provided with the additional information that it is black in color but even without this information we can easily identify the car because it has to be car number five.

The second sentence just takes you to the fifth black car; now this car could be the fifth car (in case the first four cars are also black) or it could be the tenth car or the twentieth car, in fact it can be at any number as long as it satisfies the criterion of being the fifth black car.

Thus remember that on the GMAT 'which' is only explanatory and is not needed to identify the subject of the sentence; in fact you can easily remove the phrase starting with 'which' and yet the meaning of the sentence would remain unchanged whereas 'that' is necessary to identify the subject of the sentence and cannot be done away with.

To make it even easier, remember the following 2 rules for 'which' to be correct on the GMAT:

1. It should always come after a comma
2. It must refer to the noun that comes immediately before the comma.

In case either of these conditions is not satisfied there's a pronoun error in the sentence and needs to be corrected.

For example let's modify the above example a little:

The fifth car in the row, which is black in color, belongs to Jack.

Now even though 'which' is coming after a comma the noun immediately before the comma is 'row' but 'which' is referring to the 'black car' and not to the 'black row'. Hence there is a pronoun error in the sentence since 'which' has an incorrect referent.

The only exception to the above rule is when 'which' is preceded by a preposition such as 'in which', 'of which', 'from which', etc. in which case you do not need a comma before 'which'.

For example consider this sentence:

The group of which I am a member has been dissolved

In this sentence even though there is no comma the use of 'which' is correct since it follows the preposition 'of'.

The other important relative pronouns are Who, Whom and Whose, all of which also cause a lot of confusion on the GMAT but we'll discuss these in a subsequent post.

To Sum it up:

1. 'That' & 'Which' are relative pronouns
   
   
2. 'That' is needed to identify the subject of the sentence
   
   
3. 'Which' is correct only if it comes after a comma and refers to the noun that comes immediately before the comma
   
   
4. Exception to the above rule - when which comes after a preposition (of which, in which, etc.)