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.
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