- Why are you crying? (asking for a reason)
- What do you want? (asking for a non-personal object)
- Who is this? (asking for a personal subject)
Who and whom are pronouns. They act as subject or object in their clauses. When, where, why and how act as adverbs. What, which and whose can be pronouns or determiners.
Question words normally come at the beginning of their clauses. When a question word is the subject, it comes before the verb and do cannot normally be used.
- Who (subject) said that? (NOT Who did say that?)
- Who (object) did you invite?
Search the Dictionary of Practical English Usage