“Which” or “that”? That question, which has been on my mind lately, is more easily answered than you’d think. If you can tell what’s being referred to without the words “which” or “that,” use “which.” If you can’t, use “that.”
And that’s that.
For example, here‘s an instance where you can remove the “which” phrase and the original meaning of the sentence is still abundantly clear: “'Avatar,' which was in 3-D, will sweep the Academy Awards.” Lose the phrase between the commas – “which was in 3-D” – and you still understand the sentence. "'Avatar' will sweep the Academy Awards.”
But read this: “The movie that swept the Academy Awards was in 3-D.” Remove the “that” phrase – “that swept the Academy Awards” – and you’re left with “The movie was in 3-D,” which wasn’t the point I was making at all.
A few tips to remember:
Commas, which cut out the fat,
go with which, never with that!