Who is a subjective-case pronoun that can be used only for subjects and subject complements. Whom is an objective-case pronoun and can be used only for objects. The pronouns who and whom appear primarily in subordinate clauses or in questions.

In Subordinate Clauses:

‘Who’ is the subject of is also editor-in-chief of the college newspaper.

‘Whoever’ is the subject of may be willing to donate time and money to a worthy cause. The entire subordinate clause whoever may be willing to donate time and money to a worthy cause is the object of the preposition to.

Examples:

‘Whom’ is the direct object of the verb met. This becomes clear when the clause is restructured to read: she had met whom at a conference.

In Questions:

Deciding whether to use who or whom in a question depends on the word’s function within the question.

Whom is the direct object of the verb will ask. This becomes clear if the question is restructured: He will ask whom (for example Anna, Maria, Cathy, etc.) to the prom.