为什么用what happened 而不是 what happen

为什么是What happened?而不是 What did happen? what happened 为什么不可以用 what did happen 为什么：他发生了什么?是what happened to him 而不是what did he happen?那为什么又 what happened/what was happened/what is happened/what happen what happened to him yesterday.为什么要用happened?happen可以吗 what happening?如果错了,解释以下what happened?为什么不能改为what did happen he walk over to see what happened...什么时候用what is happen 什么时候 what happen和what happened有什么区别 what happen to you还是what happened to you? What happened in that class probably reflects what is happen what happened 和what did happen的区别 what happen to sb 还是 what happened to sb