Demonstratives

[MauricioBra]

Hi,
My question is:

Do I say:

What happened? or What did happen?

And depending on the answer,why?

Thanks,

Mauricio.

[rewboss]

Normally, it's simply "What happened?"

Normally, when we construct a question, we need a helping verb like "do":

Peter made a cake.
What did Peter make?

However, there is one important exception: when the question word (what, who, which, etc) is also the subject.

Consider:

What did Peter make?
Answer: Peter made a cake.

Here, the question word is the object.

Who let the dogs out?
She let the dogs out.

Here, the question word is the subject, so we don't need to use the auxiliary verb "do" to make a question.

In your example, the pattern would be:

What happened?
Something bad happened.

Now for something a little more advanced. I said we don't need to use "do". But we can if we want to emphasize the positive nature of the sentence.

Take this dialogue:

A: I expected the machine to work, but nothing I expected actually happened.
B: So what did happen?

Here, A has explained what did not happen. B wants to know what happened instead, so he uses the verb "do" to emphasize this contrast.

Normally, you wouldn't use "do" here. But if you want to show a contrast, you can.