when 或 while

一般上，许多人多把 when 和 while当着同样的意思，并可相互对换来使用。

但这两个字实际有不同点

When I was walking,  I heard someone screamed. 是错的。

When I was walking 是动作在持续中

I heard someone screamed 是突发性的。

正确的语法是把 when 用在动作持续中的句子。

When I heard someone screamed , I was walking.

或者，把when 换掉，用while 来代替是可以的。

While I was walking, I heard someone screamed。

看看以下的几个例子！

While he was sleeping, I came to see him.

When I came to see him, he was sleeping.

While we were having breakfast, someone knocked the door,

When someone knocked the door, we were having breakfast.