Not a lot of people are really interested in this anymore, but here it is!
There are two reasons that people have trouble with who and whom.
The first reason is that one does not hear these used correctly. Have you heard the word whom used this week? this month? We have simply lost the correct use of these words.
Secondly, when the who/whom choice has to be made, the word itself is usually not in the "right" place in the sentence to allow your ear to help you; that is, the who/whom are generally used at the beginning of the unit they are part of. It is a question word; so it begins the sentence. It is a pronoun that introduces a clause; so it is at the beginning of the clause.
Who/Whom are you referring to?
(You are referring to --- .)
Who/Whom does he intend to call?
(He does intend to call --- .)
John is the one who/whom everyone expects will win the election.
(...everyone expects --- will win the election)
The English rule says that one uses who when it is nominative case and whom when it is objective. However, that doesn't always translate into the correct answer as many people do not understand all of the differences between nominative and objective.
So this is what I recommend if you want to begin to even think about using these correctly:
1. Turn the sentence/clause around so that it is in standard word order.
2. Put in he/she or him/her. Which one fits? If he/she fits, it is who; if him/her fits, it is whom.
This will work for many of the uses, and it works because your ear tells you which is correct. There are a few sophisticated uses that might cause you some trouble.
This is a start. If you want some exercises to do, let me know. If you want more information, let me know.
Happy punctuating!
Margie