W

WHO and THAT
Wrong: We'll pay you $50 for every person that joins.
Correct: We'll pay you $50 for every person who joins.
Tip: People are always who, not that.

WHO and WHOM
Wrong: Who pays who?
Correct: Who pays whom?
Tip: Whom is the objective case of who.

WHO'S and WHOSE
Wrong: Whose the guy who's face is on the dollar bill?
Correct: Who's the guy whose face is on the dollar bill?
Tip: Who's is a contraction of who is, and whose is the possessive case of who.

WHOLE NOTHER
Wrong: That's a whole nother thing.
Correct: That's something else entirely.
Tip: Let's pretend we've advanced beyond the Neanderthal era, shall we?