be
I have sent 15 emails.
How many emails have you sent?
a subject question with PRESENT SIMPLE
EX. who works here? what makes that noise?
David is.
Who is in class?
a subject question using Present Perfect
Who has done it?
No, this weekend I´m going to stay at home.
Are you going to go out this weekend?
A subject question using Simple Past
who went first? who called you?
A subject question with WHAT
what happened here? what caused the leak?
A subject question with WHICH
which car arrived first? Which team scored? Which bus goes to Tottus?