When do I use "that" versus "which"?

Use that for information the sentence needs. Use which for an aside you could remove.

Is it "the book that I read" or "the book which I read"?

In edited American English, "the book that I read." Which sounds natural in British English.

Quick answer Canonicalizes to That vs. Which

Do you put a comma before "which"?

Q: Do you put a comma before "which"?

Usually yes — which introduces extra information, and that kind of clause takes commas on both sides. If the clause is essential to the sentence's meaning, swap which for that and drop the comma. If the clause is an aside ("the car, which was blue, stalled"), the commas stay. One rule of thumb: which can be whisked away with its commas; that cannot.

i · AnswerOne line, no lecture

Usually yes — which introduces extra information, and that kind of clause takes commas on both sides.

ii · A little moreWhy this is the one to keep

If the clause is essential to the sentence's meaning, swap which for that and drop the comma. If the clause is an aside ("the car, which was blue, stalled"), the commas stay. One rule of thumb: which can be whisked away with its commas; that cannot.

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