1、当某人正在做某事,这时发生了什么:
sb. be doing sth when...
例:I was doing eighty when I overtook you.
2、当某人刚做完某事,这时发生了什么:
sb. had just done sth when...
I had just finished my report when someone knocked at the door.
3、当某人正要做某事,这时发生了什么:
sb. be about to do sth when...
One day, I was about to do some shopping when he telephoned me.