Do you put a comma before "which"?

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

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

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

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

Use that for information the sentence needs. Use which for an aside you could remove. The test: can you lift the clause out without breaking the meaning? If yes, it is a which clause, set off with commas — "my bike, which is red, got a flat." If no, it is a that clause with no commas — "the bike that I bought yesterday."

i · AnswerOne line, no lecture

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

ii · A little moreWhy this is the one to keep

The test: can you lift the clause out without breaking the meaning? If yes, it is a which clause, set off with commas — "my bike, which is red, got a flat." If no, it is a that clause with no commas — "the bike that I bought yesterday."

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