What did happen?这句话对吗?为什么?请祥解

动词 v. 发生
to take place
· Jack is telling the teacher what happened.
杰克正在报告老师刚发生的事.
· What happened next?
后来发生了什么事?
· Something has happened to the train.
火车出事故了.


动词 v. 碰巧
to do something by chance
· It happened to be a fine one that day.
那天碰巧天气好.
· I happened to see him this morning.
今天早上我碰巧看见他.
· As it happens,I have left the note book at home.
不凑巧,我把那本笔记本放在家里了.
· I happened to be going that way too.
我正好也往那边走.
· Do you happen to know her telephone number?
你知道她的电话号码吗?
· It happens that his sister is a closed friend of mine.
碰巧她姐姐是我的一个好朋友.
· It so happened that both of them were there.
正好他们两人都在那儿.

what 后是陈述语序 所以不对

错！
应该是 What happened?
也可以说 What happened to you/ her /him?

不对 可以写成“what happened？”译为“发生了什么？”

是对的,did是用来强调当时到底发生了什么

是对的啦。加did是为了强调当时到底发生了什么。举例来说，911时掉在宾州的那架93号飞机上到底发生了什么事大家还是不清楚，所以今年911时，各家报纸的新闻标题都是：
WHAT DID HAPPEN TO FLIGHT 93?
这个用法网上有大把的例子。

应该是 What happened?