What happen？主要是口语中使用。
What‘s happening？是比较正式的。
两者时态不同，意思都一样。What happen？常用于what happen to这个句型。用于一般现在时。what happen/happens
一般现在时态，都加to。what's happening?发生了什么?(正在发生的事情)
例句
I will back you up no matter what happen to you.无论发生什么我都会支持你。
Tell me word for word what happened to you.详细的告诉我发生什么了。
You look down. What happened to you?你看起来很颓废，出什么事了？
What happened to you when you started fishing ?当你开始钓鱼的时候发生了什么?
It's not what happens to you, but how you react to it that matters.重要的并不是你身上发生了什么事,而是你对发生的事情做出的反应。
Lots of things are happening here.许多事情正在这里进行着。
There have been strange happenings here lately.这里近来发生了一些怪事。
What is happening here is happening to Britain as a whole.在这里发生的事情也同样发生在整个英国。
想问“发生什么”，其他的说法：
what happened? 发生了什么事 ？
what's happening? 什么正在发生?
what has happened?(也可以缩写成what's happened?)已经发生了什么？