Whose or who's: which one do you need?

Who's is a contraction meaning who is or who has: who's coming, who's got the keys. Whose is the possessive form: whose car is this, the writer whose book I read. The test is expansion. If you can substitute who is or who has and the sentence still works, the apostrophe form is correct, and if you cannot, use whose. Who's the manager expands correctly; whose office this is does not.

The reason this trips people up is identical to the its and it's problem. English marks possession with apostrophe-s almost everywhere else, so writers reasonably assume the apostrophe form is the possessive. But possessive pronouns as a class take no apostrophe, which covers his, hers, ours, yours, theirs, its, and whose. Recognizing whose as belonging to that group rather than to the noun pattern resolves it permanently.

Two adjacent points. Whose can refer to things as well as people, despite a persistent belief otherwise, so the building whose roof leaks is perfectly standard and the alternative of which the roof leaks is stiff and generally worse. And spellcheck will not catch a wrong choice here, since both are valid words, which is why the error appears in published writing regularly. The same is true of the other common homophone pairs worth checking deliberately: your and you're, their, there, and they're, and to, too, and two, all of which follow from the same fact that sound gives you no information about spelling in English.