In order to understand the difference between “which” and “that,” first you need to understand the difference between a RESTRICTIVE CLAUSE and a NON-RESTRICTIVE CLAUSE.
The simple rule is to use “that” with a restrictive element and “which” with a non-restrictive element.
Don’t worry, it’s not so complicated.
Restrictive Clauses and Nonrestrictive Clauses
A restrictive element is part of a sentence you can’t leave out because it specifically restricts the noun.
Here’s an example:
“I’m allergic to sweets that contain chocolate.”
The words “that contain” restrict the kind of sweets we’re talking about.
Without those words, the meaning of the sentence would change.
Without them, we’d be saying that I’m allergic to all sweets, not just the ones with chocolate.
Note: Restrictive elements aren’t surrounded by commas.
Here’s another example:
“Alarm clocks that buzz too loud drive me crazy.”
I can’t leave out the words “that buzz too loud” because then I’d be saying all alarm clocks drive me crazy, not just the ones that buzz too loud.
On the other hand, a non-restrictive element is something that can be left off without changing the meaning of the sentence. A nonrestrictive element is simply additional information.
“Baccarat crystal cups, which are usually so expensive, are on sale at Bloomingdale’s.”
Leaving out the words “which are usually so expensive” doesn’t change the meaning of the sentence. With or without the words “which are usually so expensive,” we know that they are on sale.
Note: Nonrestrictive elements are surrounded by commas.
Here’s another example:
“Black cats, which are thought to bring bad luck, do not deserve to be ill-treated.”
We could throw out the words “which are thought to bring bad luck” and the meaning of the sentence wouldn’t change. Black cats would still remain undeserving of ill-treatment.
The words, “which are thought to bring bad luck,” are just extra, meaning they’re non-restrictive, surrounded by commas, and “which” is the right word choice.
Note that the pronoun “that” can ONLY be used for RESTRICTIVE CLAUSES.
Wrapping up:
Choose the relative pronoun “that” when you CAN’T get rid of the clause:
“Alarm clocks that buzz too loud drive me crazy.”
and the relative pronoun “which” when you CAN get rid of the clause:
“Baccarat crystal cups, which are usually so expensive, are on sale at Bloomingdale’s.”
