◊ asking for information
You use who when you are asking about someone's identity. Who can be the subject, object, or complement of a verb. It can also be the object of a preposition.
Who invited you?
Who are you going to invite?
Who are you?
Who did you dance with?
Note that when who is the object of a verb or preposition, it is followed by an auxiliary verb, the subject, and then the main verb. Note also that when who is the object of a preposition, the preposition must go at the end of the clause. You do not use it in front of who.
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Whom is a formal word which is sometimes used instead of `who'. Whom can only be used as the object of a verb or preposition.
Whom shall we call?
By whom are they elected?
Note that when whom is the object of a preposition, the preposition must go in front of whom. You do not use it at the end of a clause.
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◊ used in reported clauses
Who is often used in reported clauses.
She didn't know who I was.
Have you found out who Hegel is yet?
For more information, see entry at ↑ Reporting.
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◊ used in relative clauses
He's the man who I saw last night.
Joe, who was always early, was there already.
...two girls whom I met in Edinburgh.
...Lord Scarman, for whom I have immense respect.
In relative clauses, you can use either who or which after a collective noun such as `family', `committee', or `group'. After who you use a plural verb.
There are a large group of people who qualify by reason of income and social habits.
After which you use a singular verb.
He is chairing a scientific group which has set itself the task of preventing liver cancer.
◊ WARNING
\Useful english dictionary. 2012.