Question

Use "that" with essential (restrictive) relative clauses — clauses that define or restrict which specific person or thing you mean and are not set off by commas. Use "which" with nonessential (nonrestrictive) relative clauses — clauses that add extra information about something already identified and are enclosed in commas. The difference: removing a restrictive clause changes which item is meant, while removing a nonrestrictive clause only removes additional detail. Examples: "Books that have torn pages are discounted." (restrictive) versus "My book, which has torn pages, belonged to my sister." (nonrestrictive).

Answer 1

Got it — that’s a good rule. Brief summary and a few useful notes:

Rule
- Use that with essential (restrictive) relative clauses — no commas; the clause identifies which person or thing you mean.
- Use which with nonessential (nonrestrictive) relative clauses — set off with commas; the clause adds extra information about something already identified.

Quick test
- Remove the clause. If the sentence’s referent changes, the clause was restrictive (use that). If the referent stays the same and only extra detail is lost, the clause was nonrestrictive (use which).

Examples
- Restrictive: "Books that have torn pages are discounted." (Only those books.)
- Nonrestrictive: "My book, which has torn pages, belonged to my sister." (My book is already identified; the torn pages are extra info.)

Additional notes
- Use who/whom for people (not which). Some writers allow that for people in informal restrictive clauses, but many style guides prefer who/that for people and that only for things.
- Which is normally used with commas; that is not. Avoid using that in nonrestrictive (comma) clauses.
- Prepositions: which can follow a preposition in formal style ("the book to which I referred"); in informal style you can end the clause with a preposition and use that ("the book that I referred to"). Adjust for formality.
- British English sometimes uses which in restrictive clauses more freely than American style; follow the style guide you’re using.

If you want, I can revise sample sentences or proofread a paragraph to apply these rules.