1. Who vs. Whom
For additional instruction refer to the Grammar module and to Grammar Girl:

2. Who/Whom Who and whom are pronouns that take the place of a noun.
Use who when referring to the subject of a clause.
Use whom when referring to the object of a preposition.
Remember:
The subject is the person who is doing something. The object is the person who is having something done to him or her.

3. Tip
When asking a question in which you are deciding whether to use who or whom, ask yourself whether the question can be answered with he or him.
He = who
Who is going? He is.
Him = whom
Whom are you going with? I’m going with him.
If you can answer the question with he, then you should use who in the question.
If you can answer the question with him, then you should use whom in the question.

4. Examples Who/Whom took out the trash? Who/Whom does this belong to?
He took out the trash.
Who/Whom does this belong to?
It belongs to him.
That’s the person who/whom chased me.
He chased me.
I don’t know who/whom the story is about.
The story is about him.
Use who only to refer to humans. Use that to refer to nonhumans, as in:
That’s the dog that chased me. It chased me.