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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。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。when可以用于表示“一……就……”的句型中(指过去的事情)。
一、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。
① Why do you want a new job when you’ve got such a good one already?(get为短暂性动词)你已经找到如此好的工作，为何还想再找新的?
②Sorry，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。
③Strike while the iron is hot.(is为延续性动词，表示一种持续的状态)趁热打铁。
④ The students took notes as they listened.(listen为延续性动词)学生们边听课边做笔记。
二、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 phone call，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。
①As the time went on，the weather got worse.(as表示“随着……”之意)
②The 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。