when在引导状语从句时，意思是“当……时候”，所以引导的状语从句一定是时间状语从句；
when可以和延续性动词连用，也可以和短暂性动词连用；而while和as只能和延续性动词连用；
例如
Sorry，I was out when you called me．
（call为短暂性动词）对不起，你打电话时我刚好外出了。
when从句的谓语动词可以在主句谓语动作之前、之后或同时发生；while和as从句的谓语动作必须是和主句谓语动作同时发生。
1．从句动作在主句动作前发生，只用 when。
①When he had finished his homework，he took a short rest．（finished先发生）当他完成作业后，他休息了一会儿。
②When I got to the airport，the guests had left．（got to后发生）当我赶到飞机场时，客人们已经离开了。
2．从句动作和主句动作同时发生，且从句动作为延续性动词时，when，while，as都可使用。
①When ，While ，As we were dancing，a stranger came in．（dance为延续性动词）当我们跳舞时，一位陌生人走了进来。
②When ，While ，As she was making a phonecall，I was writing a letter．（make为延续性动词）当她在打电话时，我正在写信。
3．当主句、从句动作同时进行，从句动作的时间概念淡化，而主要表示主句动作发生的背景或条件时，只能用 as。这时，as常表示“随着……”；“一边……，一边……”之意。 ① As the time went on，the weather got worse．（as表示“随着……”之意）
② The atmosphere gets thinner and thinner as the height increases．随着高度的增加，大气越来越稀薄。
③As years go by，China is getting stronger and richer．随着时间一年一年过去，中国变得越来越富强了。
④The little girls sang as they went．小姑娘们一边走，一边唱。
⑤The sad mother sat on the roadside，shouting as she was crying．伤心的妈妈坐在路边，边哭边叫。
4．在将来时从句中，常用when，且从句须用一般时代替将来时。
①You shall borrow the book when I have finished reading it．在我读完这本书后，你可以借阅。
②When the manager comes here for a visit next week，Ill talk with him about this．下周，经理来这参观时，我会和他谈谈此事。
when用于表示“一……就……”的句型中（指过去的事情）。
sb．had hardly（＝scarcely） done sth．when．．．＝Hardly ， Scarcely had sb．done sth．when．．．
①I had hardly ，scarcely closed my eyes when someone knocked at the door．＝Hardly ， Scarcely had I closed my eyes when someone knocked at the door．
我刚一闭上眼，就有人在敲门了。
②I had hardly ，scarcely entered my room when the telephone rang．＝Hardly ，Scarcely had I entered my room when the telephone rang．
我刚一走进房门，电话就响了。
