一： as/were doing ...... when。 是指当后者发生的时候，前者正在做某事。 一般前面用
过去进行时，后面用
一般过去时，后者也可以用过去进行时， 即后者可以是瞬间动作或持续性动作。
例句:
1.He was reading when the bird knocked upon the window.
2.They were talking about the car accident when the teacher came into the classroom .
3.I was doing my homework when the phone rang.
4.Tom was playing football in the garden when his father was looking for him everywhere.
二： be about to do.....when。 当后者发生的时候，前者正要做某事。 后者常用一般过去式，即后者需要是瞬间动作。例句如下：
1. He was about to leave the office when her wife called him.
2. we are about to go outside when my father opened the door.
三: had just done......when。 当后者发生的时候，前者刚做完某事。
后者常用一般过去式，即后者需要是瞬间动作。例句如下：
1. I had just finished my homework when mother came back.
2. She had just washed all clothes when her son fell into a pond.