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 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。
