when,while,as都可以引导时间状语从句,但用法区别非常大,具体而言:
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。
①Why 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。
④The 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。