The question of whether to use "that" or "which" is one of the easiest grammar rules to master:
Use "that" when the subsequent clause is essential to the meaning of the sentence.
Use "which," preceded by a comma, when the subsequent clause is not essential - you could stick a period where the comma is and the meaning of the sentence would not be altered.
Here are some examples with the essential and non-essential clauses underlined:
NON-ESSENTIAL CLAUSES: I have two rugs: a red one, which was cheap, and a green one, which was very expensive.
We know these are non-essential clauses because you can get rid of them and the meaning of the sentence is not altered. I have two rugs: a red one and a green one.
ESSENTIAL CLAUSE: I have two rugs, but I'm going to get rid of the one that was cheap.
See the difference? If I get rid of "that was cheap," this sentence loses vital information. That means this clause is essential.
One of the things that makes this rule easy to remember is that people rarely have trouble getting this right when they're speaking. When the clause is essential, there's no pause - no comma - before you say it. When it's non-essential, you pause automatically. The comma goes with the one that has the pause.
© Holly A. Heyser
