Who or whom?

pronoun What person or people; which person or people; asks for the identity of someone; used in a direct or indirect question

You're getting married‽ Who to? (direct question)

noun A person under discussion; a question of which person

determiner whose

Who phone just rang?

pronoun Honorific alternative letter-case form of who, sometimes used when referring to God or another important figure who is understood from context

pronoun With antecedent as subject

That's the man who works at the newsagent. (defining)

pronoun With antecedent as object: whom

That's the man who I saw earlier

pronoun Whoever, he who, they who

Who dares wins

pronoun informalAlso used with names of collective nouns that are groups of people, especially singularly-named musical groups or sports teams

Who are the Miami Heat?

pronoun As the object of a verb

Whom did you ask?

pronoun Honorific alternative letter-case form of whom, sometimes used when referring to God or another important figure who is understood from context

pronoun As the object of a preposition

To whom are you referring?

pronoun Used to refer to a previously mentioned person or people

That is the woman whom I spoke to earlier. (defining)

pronoun The person(s) whom; whomever

To whom it may concern, all business of John Smith Ltd. has now been transferred to Floggitt & Runne

pronoun informalAlso used with names of collective nouns that are groups of people, especially singularly-named musical groups or sports teams

Whom was Lemmy bassist for? Motörhead!