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Why we say"What happened ?" instead of "What did happen"?

I have asked this question in Y Q and they said "What happened" is the correct one.

Why ?

Do you have any rule to follow for this kind of situation please?

Thanks a lot.
Comments 
Don't add do, does, or did when the question word (what in this case) is the subject of the sentence.

What did Jack say? Jack is the subject. What is not the subject.

What happened? What is the subject.

Who does he look like? he is the subject. Who is not the subject.

Who looks like me? Who is the subject.

Which forks do you place next to the plate? you is the subject. Which forks is not the subject.

Which forks go next to the plate? Which forks is the subject.

CJ
austere125rivers"What did happen"?
Actually, this is used for emphatic expression (surprise, shock, disbelief).

So if that didn't happen, then what DID happen?
Site Hint: Check out our list of pronunciation videos.
You are the best! Thank you so much Calif !All your anwers are backed with proofs.

Do you have any grammar books ??
austere125riversDo you have any grammar books ?
See my response to your other post. I'm not in a position to advise anyone on this topic. Sorry.

CJ