what happened 和what did happen的区别

what happened 和what did happen的区别 what happen和what happened有什么区别 请问：happen的用法.eg:what happened,what's happen的区别 what happened 为什么不可以用 what did happen 为什么是What happened?而不是 What did happen? what happened ?和what has happened ?的区别 What has happened和What happened的区别 what happened/what was happened/what is happened/what happen What happened?中的happen不是动词吗?怎么不是写成What did happen? What happened to you与how did it happen?的用法 what happening?如果错了,解释以下what happened?为什么不能改为what did happen what happened 和WHAT IS HAPPEND的区别