Understanding the difference between the pronouns “who” and “whom” is very easy if you simply follow the rules of usage. “Who” is the subjective or nominative case and “whom” is objective. Knowing this, the only thing left to do is to determine which case is appropriate for each use of the pronoun.
Knowing the case that should be used may be the trickier part of this problem. To do so, you have to be able to tell the difference between a subject and an object in a sentence. If the pronoun is used as a subject, then the correct form will be “who”, the subjective form. For example, “The man who called me yesterday is tall.” In this sentence, “who” is used as the subject of the clause, identifying the one performing the act of calling.
A good test of correct use is to substitute the words “he” and then “him” for “who.” If “he” sounds right, then “who” is probably correct. If, on the other hand, “him” seems to fit, then “whom” is more likely correct. You just use “he” to test “who” and “him ” to test “whom.” Follow the “m”, in other words. Almost everyone can tell that “he called” sounds better than “him called.”
If the pronoun who or whom is being used to represent the person performing the action, then it should be “who” with no “m.” When the pronoun is the receiver of the action, it will be “whom”, with the “m.”