that / which in relative clauses
Restrictive versus nonrestrictive.
Use that to restrict the noun—to tell which one you mean—and do not set the clause off with commas: the brief that she filed. Use which for a nonrestrictive clause, and set it off with commas: the brief, which she filed on Monday, is short. Do not use the brief which she filed for the restrictive sense.
Examples
Usethe crate that jammed the gate
Usethe crate, which jammed the gate, is still there
Avoidthe crate which jammed the gate