当谈论两个事件同时发生时,可以用when/while/as.
在谈论当某件事正在发生时,另一件事发生了,就用when/while/as引出较长的背景,表示该背景先于另一事件发生,并且可能还会延续下去.例如：When/While/As I was walking down the street I noticed a police car in front of number 37.
这种时间从句也可以后置.
在引出较长的背景时,多用when/while,而且该从句的谓语动词是过去进行时.如：
John arrived when/while I was cooking lunch.
When的用法比较灵活,它表示的时间可长可短,如：
John arrived when I was cooking lunch.也可以说When John arrived, I was cooking lunch.