happen是一个动词，一般都是指已经发生了的事情，所以习惯用What happened这样的过去式。如：What happened? You seem nervous.发生什么啦？你似乎很紧张。
happen的用法例句：
1.Remember, keep a positive attitude and good things will happen.
记住：保持乐观的心态，好事自然会发生。
2.Do not wait for good things to happen to you. You need to walk towards happiness.
不要等待好事降临，你要向幸福进发。
3.This is such an unforeseeable situation that anything could happen.
这种情况很难预测，什么事都有可能发生。
4.I go all tingly when I think of what might happen.
我一想到可能发生的事就满心欢喜。
5.He couldn&39;t help worrying obsessively about what would happen.
他禁不住时刻担心将会发生的事情。
6.When events happen that shake these beliefs, our fear takes control.
当一些事情的发生动摇这些信念时，恐惧就会占据我们的心灵。
7.Lucy shut her eyes so she wouldn&39;t see it happen.
露西闭上了眼睛，这样就不会看到它发生了。
8.Deep inside the awareness was stirring that something was about to happen.
内心深处隐隐感觉到有什么事即将发生。
9.She wondered what would happen if her parents found her.
她不知道如果父母找到自己会有什么结果。
10.To start with, where and when did it happen?
这事发生在何时何地?
