The basic rule is to use THAT before a restrictive clause and WHICH before everything else. But what does that even mean? Here’s the breakdown:
THAT: a restrictive clause
A restrictive clause is a part of a sentence that you CANNOT remove because it specifically “restricts” some other part of the sentence.
Here’s an example: “Parakeets that talk often induce headaches.”
The words “that talk” restrict the kind of parakeets you’re talking about. Without them, the meaning of the sentence would change. Without them, you’d be saying that all parakeets induce headaches, not just the parakeets that talk. Note: you don’t need commas around the words “that talk.”
WHICH: a non-restrictive clause
A non-restrictive clause is something that can be left out of a sentence without changing the meaning of the sentence. You can think of a non-restrictive clause as simply additional information.
Here’s an example: “Macaws, which are colorful, are bigger than parakeets.”
In this particular sentence, leaving out the words “which are colorful” doesn’t change the meaning of the sentence. (Also note that the phrase is surrounded by commas. Non-restrictive clauses are usually surrounded by, or preceded by, commas.)
Now, let’s look at the compare the above sentences with THAT and WHICH.
Parakeets that talk often induce headaches.” (Restricted to parakeets that can talk, not all parakeets.)
Parakeets, which talk, often induce headaches.” (Includes all parakeets; can remove “which talk.”)
“Macaws that are colorful are bigger than parakeets.” (Restricted to macaws that are colorful, not all macaws.)
“Macaws, which are colorful, are bigger than parakeets. (Includes all macaws; can remove “which are colorful.”)
Still not sure? Think about this: if removing the clause will change the meaning of the sentence, use THAT. On the other hand, if removing the clause doesn’t change the meaning of the sentence, use WHICH.
Memory trick: If you still can’t remember, try memorizing “YOU NEED THAT’ (i.e., THAT clauses are clauses you NEED in a sentence.)