The dilemma of choosing who or whom is one that haunts many an English-speaking person, while other languages do not make this distinction.
In addition to the explanations given above, there is another little method of determining whether one uses who or whom in clauses, the situation which causes most of the dilemmas. But by simply determining whether there is already a subject in the clause, the writer will know whether to use nominative who or the objective whom. Here are examples:
1. Stephen Liner is the businessman (who,whom) _______I met at the airport last week.
In the relative clause, I is a nominative case pronoun, so I is the subject of this clause. Since I is already the subject, the writer must use whom as it is the object of the verb met:
Stephen Liner is the businessman whom I met at the airport last week.
[the error of who is often made here because who precedes I, but the order is not the important element; it is the syntax of the words that matters.]
2. Marianne is the hostess (who,whom)_______will seat you at the appropriate table.
In this sentence, the pronoun you is the object of the verb will seat, so there is not a subject for this clause. Therefore, the position is open for who:
Marianne is the hostess who will seat you at the appropriate table.
This is a relatively straight forward problem, but it does require a little background knowledge about grammar, something that we don't push really hard these days in many English classes.
If you know what a subject and an object are, skip to the last paragraph. If you don't, try to think of the subject as someone that is going to do or perpetrate an action or be something and an object as something or someone acted upon. I pet my dog. I am the subject and the dog is the object.
So, when it comes to who and whom, it is really simple. Who is used as the subject, whom is used when it is the object.
To whom are you sending the letter? Whom is the object of send.
Who is sending the letter? Who is the subject, sending the letter.
Hope that helps.
This is a good question; and most people try to get it right but often don't because they somehow think whom is more sophisticated and use it more often than it should be used.
I'm assuming you're at least familiar with the basics of grammar and understand what an object is in a grammar discussion. Whom is used as a pronoun replacement for any object: direct object, indirect object (though not a likely usage, it's correct), or object of the preposition. That's a lot of choices, I know; so, it may be easier to remember that who is used to replace subjects and predicate nominatives. So, for example, in the following sentences, we use who:
I voted for the candidate who is stronger. (subject of the adjective clause)
Who is going to the movie? (subject of the sentence)
Who? Susie is who. (predicate nominative)
The following sentences use whom as objects:
Ask not for whom the bell tolls. (object of the preposition)
You asked whom to the dance? (direct object)
In general terms, if the word who or whom comes toward the beginning of a sentence, you would usually use who. If the word who or whom comes later in the sentence, it's generally whom. That's not always true, and that rule doesn't really work with questions (or other inverted sentences), but it's a guideline.
Thank you for at least wanting to use these correctly. Hope this helped.