- THAT should be used to introduce a restrictive clause.
- WHICH should be used to introduce a non-restrictive or parenthetical clause.
A restrictive clause is one which is essential to the meaning of a sentence – if it’s removed, the meaning of the sentence will change.
The book that you gave me last night has several pages missing.
Can you return the DVD that you borrowed last month?
A non-restrictive clause can be left out without changing the meaning of a sentence. Non-restrictive clauses are either in brackets or have a comma before and after them (or only before them if they come at the end of a sentence):
Paulo Coelho’s latest book, which my dad gave me on my birthday, has really inspired me.
Driven by desperation, Lucy even pawned her diamond wedding ring, which was also a family heirloom.