He was on the point of leaving when someone knocked at the door. 他正要走,这时有人敲门。
2. 如果谈论两个长动作,最常用的是while。
While John was sitting biting his nails, I was working out a plan to get us home. 约翰坐在那里咬指甲时,我想出了一个回家的办法。
注:while 除了当...的时候的意思之外还有另外三个意思,要区分一下状语从句类型.
3. as引导一个持续性的动作,主句中的动作与之同时进行。
As we talked on, he got more and more excited. 我们继续往下谈的时候,他越来越兴奋。
as 还引导原因状语从句.
4. 有些词语搭配要求用wehn when
比方说: be about to ...when
过去进行时...when
hardly / scarecely...when
主句出现以上表达,从句中要求必须用when.