Okay, here’s one of the biggies — even I used to have difficulty with the various forms of the word “who,” but the rules are actually pretty simple when you get right down to the basics.

“Who’s” is a contraction of “who is,” so one only uses it as a contraction for “who is.” If you replace it in a sentence with “who is” and the sentence doesn’t make sense, you’ve used it incorrectly.

“Whose” is a possessive — it indicates ownership of an object (or responsibility, in the case of “Whose fault.”

That brings us to the tricky ones: Who and Whom.

Remember the preposition discussion? Here’s a gentle reminder: A prepositional phrase is a bridge from the verb of a sentence to its object. Well, that’s a clue to who and whom. Who is the subjective form (use it in the subject part of a sentence). Whom is the objective form (use it in the object of a sentence).

So the question, “Who does this belong to?” is grammatically incorrect not because it begins with the word “who,” but because it ends with a preposition, a bridging word. It’s that bridge-to-nowhere thing. Who is actually right, because it’s the subject of the sentence (it’s the rest of the sentence that, well, has issues.) Correctly structured, the question becomes, “To whom [preposition>>objective form] does this belong?”

But who talks like that? [subjective form>>verb, no prepositional phrase.]