A short cut to "who" and "whom."
* Nominative: who
* Possessive: whose
* Objective: whom
The rule:
1.
treat the "who-clause" as a mini-sentence.
If you could substitute "he" for the who-whom, it's a "who." If you could substitute "him" for the who-whom it's a "whom."
The trick is where ellipsis has occurred or where parentheticals have been inserted and the number of people in important and memorable places who get it wrong. "Who do I see?" Wrong: I see he? No. I see "him." Whom do I see?
2.
"Who" never changes case to match an antecedent. (word to which it refers)
* I blame them who made the unjust law. CORRECT.
* It is she whom they blame. CORRECT: The who-clause is WHOM THEY BLAME.
* They blame HER=him, =whom.
* I am the one WHO is at fault. CORRECT.
* I am the one WHOM they blame. CORRECT.
* They took him WHOM they blamed. CORRECT---but not because WHOM matches HIM: that doesn't matter: correct because "they" is the subject of "blamed" and "whom" is the object.
* I am he WHOM THEY BLAME. CORRECT. Whom is the "object" of "they blame."
Back to rule one: "who" clauses are completely independent in case from the rest of the sentence. The case of "who" in its clause changes by the internal logic of the clause and by NO influence outside the clause. Repeat to yourself: there is no connection, there is no connection 3 x and you will never mistake for whom the bell tolls.
The examples above probably grate over your nerves. That's why "that" is gaining in popularity in the vernacular and why a lot of copyeditors will correct you incorrectly on this point. I'm beginning to believe that nine tenths of the English-speaking universe can't handle these little clauses.
http://www.sfwa.org/writing/chadvce.htm