what you should write. If your sentence needs a comma to achieve its precise meaning, it probably needs “which.” “Which” serves a particular identifying function, different from “that.” (A) “Take the shoes that are in the closet.” This means: take the shoes that are in the closet, not the ones under the bed. (B) “Take the shoes, which are in the closet.” Only one pair of shoes is under discussion; the “which” usage tells you where they are. Note that the comma is necessary in B, but not in A.
