as, when, while三个词都有“当……时候”，但是三者的用法确是有很大区别。大家要注意它们的区别，这个知识点也是中考英语的常考知识点，一定要引起重视。
1.as：
①多用于动作发生时间较短时，常有“正当……之时” 之意
He fell off the bike as she got down.
②表“边……边……”或“随着……”之意时常用as
SHe whistled as she rode on.
2.when表示“当……的时候”，引导的从句即可以接延续性动词也可接非延续性动词
When We arrived, it was already midnight.
从句和主句的动作同时发生，但因为从句动作arrived是非延续性的,表示的是“一瞬间”，此时不可用while，但可用as替换。
When the clock struck eleven, all the lights wentout.(从句的动作在主句之前发生)
When he ran to the stop, the bus had gone.(从句动作在主句动作之后发生)
He read a newspaper when he was waiting for a bus.
从句动作和主句动作同时发生，而且从句动作waiting是延续性的，表示的是“一段时间”，此时可用while替换。
3.while：
while意思是during the whole of the time that,指在“一段时间”里，所引导的从句中的动词是延续性的，可译为“在……期间， 在……之时”
She visited a lot of places while she stayed in U.S.
He fell asleep while he was watching TV.
注意：when和 while所能接的动词范围不同
练习：
I was walking onthe street______ a dog ran out.
A. when B. as C. whileD. since
【答案】A
