Use "that" in a restrictive clause, to specify a particular thing you are talking about, and use "which" in a nonrestrictive clause, to provide additional information about something. The "which" clause is not essential to the sentence and can be removed from the sentence without changing the meaning.
Examples:
- Restrictive clause: The car that was in the driveway was damaged by the falling tree. Here, "that" specifies the particular car that was in the driveway, singling it out from any other cars. It could be that the car that was in the driveway was damaged, but the cars parked in the street were fine.
- Nonrestrictive clause: The car, which was in the driveway, was damaged by the falling tree. Here, "which" is used to add the nonessential information that the car happened to be in the driveway. If the "which" clause is removed, you get "The car was damaged by the falling tree," and that is the essential meaning of the sentence.
