when （at or during the time that ）既可以表示在某一点的时候, 又可表示在某一段时间内, 主 句与从句的动作或事情可以同时发生也可以先后发生。 It was raining when we arrived.（指时间点）When we were at school, we went to the library every day.（在一段时间内）
while意思是“当……的时候”或“在某一段时间里”。主句中的动作或事情在从句中的动作或事情的进展过程中发生, 从句中的动词一般要用延续性动词。在when表示a period of time时, 两者可以互换。 Please don’t talk so loud while others are working.
He fell asleep while/when reading. Strike while the iron is hot.（用as或when不可, 这里的while意思是“趁……”
（2）引导时间状语从句的连接词除上述外还有：①till, not … until …, until, before, since
Don’t get off the bus until it has stopped. w*w*w*k*s*5*u*c*o*m
He waited for his father until（till）it was twelve o’clock.
It will be five years before he returns from England.
②hardly / scarcely … when, no sooner … than, as soon as once表示“一……就”
As soon as I have finished it , I’ll give yu a call.
Once you show any fear, he will attack you.
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