Questions with who, who(m), and what

(a) Who came? Someone came
(b) Who (m) did you see? I saw someone
In (a): Who is used as the subject (S) of a question
In (b): Who (m) is used as the object (O) in a question
Whom is used in formal English. In everyday spoken English, who is usually used instead of whom:
Formal: Whom did you see?
Informal: Who did you see?
(c) What happened? Something happened
(d) What did you see? I was something
What can be used as either the subject or the object in a question.
Notice in (a) and (c): when who or what is used as the subject of a question, usual question word order is not used; no form of do is used:
Correct: Who came?
Incorrect: Who did come?
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