I somehow know that the first one is the correct one. I just don't know how to justify it.
What do you think? Do we have to use an auxiliary all the time or is it correct not to in this case?
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"What happened" (past tense) is correct.
This applies to other verbs that refer to events, such as "occurred" and "transpired," and verbal phrases such as "took place."
The auxiliary "did" and present tense "happen" give the question additional emphasis, the same as asking "what really happened?" For example, if you heard two different accounts of an event, you might ask a third person, "what did happen?" to convey that more detail or clarification of the story is needed.
"What" is a pronoun, so it replaces a noun. "What" is used for forming questions. The noun that "what" replaces can either be an object or a subject of the sentence. Here are two examples:
What did you eat?
I ate a hot dog.
What ate the hot dog?
The bear ate the hot dog.
In the first conversation "what" replaces the object. Usually objects come at the end of sentences, but in a "what" question, the question pronoun comes first.
In the second conversation, "what" replaces the subject. There is no inversion and no auxiliary verb "did".
You can make an emphatic sentence
What did eat the hotdog?
Ok I lied, there was no bear. I ate it.
Using "did" suggests that the questioner doesn't believe the previous answer.
Now. The verb happen is intransitive. It doesn't have a direct object. So you form questions as in the second converstion
What happened during the earthquake?
Buildings shaking happened during the Earthquake.
(actually you would say "The buildings shook", but I wanted to show how what is standing for a subject in the question.)
Saying "What did happen" is emphatic. It suggests that you don't believe what you have heard.
What really did happen during the earthquake. I've never felt one. Was it noisy? Did any buildings fall down?