That vs Which
Does it matter? Can’t you use them interchangeably? Yes, it does and no, you can’t. Most grammar sources will tell you that is used in restrictive clauses and which is reserved for non-restrictive clauses, which is great if you know what restrictive and non-restrictive clauses are.
Here are two reasons which was the correct choice in that last sentence: A) omitting it does not change the meaning of the sentence. Omitting the section beginning with which does not alter the meaning of the sentence, and B) there was a comma before it.
Grammar Girl Mignon Fogarty uses this example: Gems that sparkle often elicit forgiveness. But: Diamonds, which are expensive, often elicit forgiveness. The first sentence cannot exist if you remove that, but the second one does fine without which are expensive.
You will often hear things like: Diamonds that are expensive often elicit forgiveness, but it’s not correct. It implies that only those diamonds that are expensive will work, but what the sentence means is that all diamonds are expensive and they all often elicit forgiveness.
So, try this sentence: The dress that she bought was very pretty. Is that the correct choice? If you said yes, give yourself a hand…sort of. It is correct as opposed to which, but I maintain, as my very first post said, that that is possibly the most overused word in the language. The meaning of this sentence does not change at all if you leave out the word: The dress she bought was very pretty. See? Works fine. But you can’t always omit it. Gems sparkle often elicit forgiveness is just silly.
Now, some of this may sound contradictory, since I just said, which is used in phrases that can be omitted. But that was talking about omitting the entire phrase, not just the word. Whenever you use that, see if the sentence works without it. If not, which may be the word you need.
But here’s the most important thing to remember about that and which: If you’re talking about a person, neither is correct. Use who. Or whom. More on that in a future post.