As a native speaker, I hear people everyday use “who” and whom” wrong, so don’t be scared to make a mistake.

The method J. Luke outlines, of reviewing the most appropriate pronoun, is how I work it out when I get stuck. Except when you're in an exam, don't panic about this. Actually, very few native speakers get this right and often find even the correct use of whom sounds strange. Many people will always use who. I don't mean you shouldn't learn it and use it, just don't worry about it too much.

"Who" refers to the subject of a sentence. "Whom" refers to the object of a sentence. If you can replace the who/whom with He/She, use 'who." If you can replace the who/whom with him/her, use "whom." Example: Who gave what to whom? He/She gave the car to him/her.

They are tricky and somewhat interchangeable, and we native speakers often mull over both options, or try both in a sentence one after the other to see which sounds better in a given context. That said, here are a couple of examples: "To whom does this belong?" "Who left this sitting here?" "It's a matter of whom she has left to bother now." "It's a matter of who's left for her to bother now." "To whom do we owe the pleasure of your company?" "Who asked you to come see us today?" Hope that helps ~