1. "Which" as a relative pronoun introducing a restrictive clause:
* No punctuation is needed. A restrictive clause is essential to the meaning of the sentence and cannot be removed without changing the meaning.
* Example: "The book which I borrowed from the library is due tomorrow."
2. "Which" as a relative pronoun introducing a non-restrictive clause:
* Use a comma before "which". A non-restrictive clause is not essential to the meaning of the sentence and can be removed without changing the meaning.
* Example: "The book, which I borrowed from the library, is due tomorrow."
3. "Which" as an interrogative pronoun:
* No punctuation is needed. This is a direct question.
* Example: "Which one do you prefer?"
4. "Which" as an adjective:
* No punctuation is needed.
* Example: "Which shirt should I wear?"
Important Note: If you are unsure whether a clause is restrictive or non-restrictive, think about whether the meaning of the sentence would change if the clause were removed. If it does, the clause is restrictive. If it doesn't, the clause is non-restrictive.