The only thing that has not been explained here is the clause. "what happened" here is a noun clause, acting as a noun. You could say the sentence above as "Did you see it?" But instead of "it" we can be more specific using a noun clause. A noun clause is a noun "What" (something) plus a verb "happened". So the subject and verb are in statement order, not question order, because it is not a question. The main question is "did you see".

I agree with David, and I thought perhaps you could be asking this question because of the "did" in the beginning. It is true that when you use "did", the main verb should be in the present, as in "it did happen." Here, the main verb is "see," so it is "did you see" (rather than "did you saw," which is wrong). The verb that follows "what" in your sentence, however, is a separate verb, so to keep all the tenses in the past, you say: Did you see | what happened? past tense | past tense I hope this helps!

Because 'happened' is past tense. What will happen to the boy? The boy will fall. What happened to the boy? The boy fell