Lena,

Your posts to the forum are very good.

I'm a native American English speaker and this one when I first read it made me shake my head as "what did happen" sounded so odd to my ears but then I started to think about it and I actually do say "What did happen." It is only for a very specific thing though, when I'm upset... say with my children and they are saying that one thing happened and I've heard a different version or two of the same story. I would indeed say (probably through gritted teeth), "So, what DID happen???" Or I might say "So tell me, what DID happen?" At that point, my girls better tell me the truth about what happened.