Pronoun agreement:

Question

How do you know when to use who or whom?

Who is a subject of a verb.
Examples:
Who let the dog out?
Pat is the person who is always late.

Whom is used as the object of a verb or a preposition.
Examples:
He asked, "For whom does the bell toll?"
Whom did you see entering the building?

Check this site for more information.
(Broken Link Removed)

Thanks

http://grammar.ccc.commnet.edu/grammar/pronouns1.htm
Scroll down to Personal Pronouns.

http://www.uottawa.ca/academic/arts/writcent/hypergrammar/pronref.html
Scroll down to Use "who," "which," and "that" carefully.

http://members.cox.net/lenco1/grammarpractice/
Click on Usage Practice at the left, then on Who/Whom.

Basically, if you can rephrase and use "him" or "her" or "them" -- then you need "whom." But if you rephrase and use "he" or "she" or "they" -- then you use "who."

The above is very general; the information in the websites above is more specific.

=)

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