What happened?のような質問（What/Whoが主語の質問）

What's up!

目的語を質問

What	did you	buy?
何を	君が
Who		invite?
誰を

主語を質問

It happened.

What	happened?
何が
Who	came?
誰が

主語を聞く場合は肯定文の形をとる

He came.
Who came?

He cooks.
Who cooks?　（sが付く、複数でも同じ）

He's working.
Who's working?

He's going to come.
Who's going to come?

It happened.
What happened?

It happens.
What happens?

It's happening.
What's happening?

It's going to happen.
What's going to happen?

WH名詞節になる

What happened?
I don't know what happened.

Who's going to come?
I don't know who's going to come?