为什么what直接加happen,而how加助动词did加happen

为什么what直接加happen,而how加助动词did加happen 为什么有时happen用助动词进行提问,而有时则不用?如：What will happen...What happened...What did happen...What was happening. what happen为什么没有助动词 But what happens if it rains?为什么happen要加sBut what happens if it rains?为什么happen要加s 为什么用what happened 而不是 what happen what happen后面要加什么词? I didn t know what was happening outside这句话中为什么happen加ing? what do you think to happen there yesterday 中TO为什么Y要加? What happenS if you lose a library book 其中Happen为什么加 What happens to trees in the different seasons?happen为什么加s don't give up the hope no matter what happens为什么happen必须加s? 关于happen的用法书上有一句是what happened?在这里,动词happened 不是应该加一个助动词did吗?而且还应该有主语呀,为什么可以直接用? what/that has happened.为什么是选what?happen后面不是不加宾语嘛?为什么不是that啊! what does he happen?为什么错 What happen what happen No one knows___in the future.A.what will be happen B.what will happen C.what will it happenA.what will be happen B.what will happen C.what will it happen D.what it will happen 为什么选B? what was happened 为什么不能用what was happen