Grammar Post: Who or Whom?

I have a difficult time knowing when to use the word "whom" as opposed to "who." Below is the answer I found in Purdue's Online Writing Lab:

This is something that trips up lots of writers. Simply stated, you use "whom" when the pronoun is occupying a position in the sentence that is in the objective case. Objective case includes objects of a preposition (to whom), direct objects (You hit whom?), and indirect objects (I should give whom the invitation?). "Who" is used in the subjective case: the subject of a sentence (Who is there?) and the predicate nominative (This is how I came to be who I am today.).

You really need to know your parts of speech in order to know when to use who or whom, which is why so many people use it incorrectly.

http://owl.english.purdue.edu/lab/owl/owlnews/042001.html