Sentence Correction

THAT vs WHICH


What's the Difference Between that and which?
After people study GMAT sentence correction for a while, they may ask about the difference between that and which. I have to say for the record, though, that it is not terribly important to know the difference between the two to get a high score on the test, but I know that some people are naturally curious about the difference (people here tend to be overachievers!).
A Tip
But first, a testmagic tip: GMAT almost always (I say almost always because I've seen two questions that did not follow this rule, but the rule was violated in all five answer choices) wants you to put a comma before which.
In other words, if you see which without a comma before it, it's probably wrong.
After this explanation, you should understand why, but for those of you
who want only the most important information, this is what you need to know.
An Example
Both of these sentences are correct in GMAT land:

Please go into the room and get me the big book, which
is mine.
Please go into the room and get me the big book that
is mine.
Yes, in GMATland, these two sentences have two different meanings.
Both of these sentences would be incorrect in GMAT land:
X Please go into the room and get me the big book which is mine. X
X Please go into the room and get me the big book, that
is mine. X
Notice the commas--that's what makes all the difference.

The Explanation
Okay, we have in English this weird idea that we need to use different grammar in an adjective clause (a.k.a. relative clause) depending on whether the information in the adjective clause is necessary to specifically identify which noun we are referring to.
For example, imagine you have one sister, and you are telling a friend that your sister is coming to visit you. Since this person is your friend, we can presume that he knows that you have only one sister.
You utter a sentence like this to your friend in GMATland:
My sister, who just graduated from college, is coming to see me.

In GMATland, since your friend (we presume) knows you well and knows that you have only one sister, this extra bit of information is considered unnecessary to identify which sister it is you are talking about. It is a sort of ‘By the way’; information; ‘My sister is coming to see me, and oh, by the way, she just graduated from college’.
Now imagine you have two, three, or even more sisters. Let's imagine that one is a college professor, another is a webmaster, and this one who is coming to visit you just graduated from college. If you're talking to your friend, and you say only ‘my sister,’; and you do not mention her name, your friend might not know which sister you are talking about. So you add that extra bit of information--my sister who just graduated from college--to identify which sister it is you are referring to.
In this situation, we have just correctly employed a very important grammar rule. Read on.
So, if the person you're talking to, or the person who's reading what you've written, needs that extra bit of information to know which noun you're referring to, we say that that extra information is non-restrictive. This word doesn't really describe the function clearly, so many teachers say that this information is ‘extra’.
On the other hand, if you need that information to know which noun you are talking about, we say that the information is restrictive. Again,
this word is not really a good choice for clarity, and many teachers use the term ‘necessary information’ instead.
Finally, just to make English a bit more difficult, we have a rule that says
we should use a comma before or after ‘extra information clauses and phrases’ but not with ‘;necessary information clauses or phrases’. The idea here is that the comma represents the slight pause in speech or change in intonation that a native speaker might use when making such an utterance.
I should point out that both that and which are relative pronouns,
i.e., they are grammatically the same, but their meanings are slightly different.
Now, let's return to our original example sentences:

Please go into the room and get me the big book, which is mine.

In this sentence, the clause which ‘is mine’ is ‘extra’ because the information ‘the big book’ is enough to identify which book it is that you want. We can assume that there is only one big book in the room.

Please go into the room and get me the big book that is mine.

In this sentence, the clause that is mine is ‘;necessary’ because
the information ‘the big book’ is NOT enough to identify which book it is that you want--it is probably the case that there are several big books in the room, so I need to add the information ‘that is mine’ to identify which book it is that I want.

More Examples
A few more examples may help:

I met with Bill Clinton, who is a lawyer.

The name Bill Clinton is enough to identify which person I'm talking
about--who is a lawyer is therefore extra information.

I met with the man who is a lawyer.

In this case, ‘the man’ is not enough information to identify which
person I'm talking about--who is a lawyer is therefore necessary information.

The Sun, which is the only star in our solar system, is the source of heat
for Earth.

Again, the name ‘the Sun’ already clearly identifies the noun; therefore,
the information in the adjective clause ‘which’ is the only star in our
solar system ‘is extra’

The star that is at the center of our solar system is called what?
In this case, since we don't have a name here, we don't know which star it is that we are referring to. Therefore, the information in the adjective clause ‘that is at the center our solar system’ is necessary.

by ERIN
_______