"whom" is a fancier, more refined version of "who."
This is comparable to using "him" instead of "he" because you think it sounds smarter. Such a practice prompts the inevitable questions: Do you really think that? Really?
Here is a STRANGELY AFFECTED AND THEREFORE MORALLY BANKRUPT SENTENCE: Clive is someone whom I always thought would win the dogsled-eating competition.
Who did you think would win the dogsled-eating competition? Clive. Who would? Him. Are you quite sure? Mm hmm, him would win it. Really? Yes, because him excellent eater of dogsleds.
Here is a CORRECTED REWRITE: Clive is someone who I always thought would win the dogsled-eating competition.
Who did you think would win the dogsled-eating competition? Clive. Who would? He would.
Dr. T's prescription: If you could correctly say he, she, we, they, etc. after turning the sentence into a question and then answering it, as above, use who.
|Dr. T helps relieve painful nominative and objective pronoun constipation.|
We all fart occasionally, but there's no need to do so while writing for public inspection. Follow Dr. T's simple prescription, and the internal pressure to write strangely affected sentences will pass.
The only time you want to use whom is when you can turn your sentence into a question, and then answer it correctly with him, her, us, them, etc. Thus: Clive is the contestant to whom the judge gave the biggest and shiniest dogsled-eating trophy. To whom did the judge give that pretty trophy? To Clive. To whom? To him! To him and no one else!