‘That’ is a restrictive pronoun, which means that it introduces information that is necessary to the meaning of the sentence and therefore does not need to be set off by commas.
‘Which’ is a nonrestrictive pronoun, which means that it introduces additional information to the sentence and therefore the phrase that it introduces is set off by commas.
That | Which |
Gives essential information; no commas | Gives added information; requires commas |
The toaster is broken. | |
The toaster that is broken is on the counter.-‘that is broken’ identifies the toaster that I am talking about, as opposed to the new toaster or the antique toaster. . . | The toaster, which is on the counter, is broken.-‘which is on the counter’ is not required information. The point of this sentence is that the toaster is broken. |
The computer doesn’t work any more. | |
The computer that my boss gave me doesn’t work any more.-‘that my boss gave me’ identifies the computer that I am talking about, as opposed to my personal computer or my sister’s computer. . . | The computer, which my boss gave me, doesn’t work any more.-‘which my boss gave me’ is not required information. The point of this sentence is that the computer doesn’t work any more. |
The clock is ticking. | |
The box that I left on your desk is ticking.-‘that I left on your desk’ identifies the box that I am talking about, as opposed to the box that is sitting in the trunk of my car or the box in the janitor’s closet . . . | The box, which I have never seen before, is ticking.-‘which I have never seen before’ is not required information. Who cares if I have never seen it before? It is ticking! |
The bottom line: use ‘that’ and no commas with a phrase that is essential to the meaning of your sentence. Use ‘which’ for phrases that are not essential to the meaning of your sentence, but don’t forget to place a comma before ‘which’ and after the last word in the phrase. | |