Who Or Whom?

[Not Insane Just Crazy]

I've always wanted to know the case of when to use 'whom' or when to use 'who'.
I know one of my English teachers probably taught it to the class, but I was never paying attention.  

[ChibiStarlet]

I think you use whom when you're talking about indirect objects, especially when using prepositions. For example, I would ask, "To whom is that addressed?" because you would reply, "The letter is addressed to Bob."

If you're talking about the subject of a sentence, you'd use who. So if I asked, "Who sent this?", I'd be okay because you would reply, "Bob sent this." The information you're looking for is the subject of the sentence, not the object.

That explanation is a little confusing, but I hope it helps somewhat! 4laugh  

[Argent Wraith]

ChibiStarlet got it, basically.

"Who" is nominative. That means you use it as a subject or after a linking verb as a subject complement
Examples:

Who knocked on the door?
("Who" is the subject.)

I can't think of a subject complement example off the top of my head. x_x

"Whom" is objective. Use it as a direct object, (Whom did she hit?) indirect object, (no example, sorry!) or object of a preposition. (For whom was that package?)

I apologize for my lack of examples. I hope this has helped in some way! You could always google it and pray to find a competent website...