Ⅰ. as 当(在)时候往往可与when 或while通用，但它着重指主句和从句的动作或事情相并发生。如：
① I saw him as he was getting off the bus. 当下公共汽车的时候，我看见了他。
② As he walked on, he felt himself getting more and more tired.他继续往前走的时候，感到越来越疲乏。
Ⅱ. when 当(在)的时候主句和从句的动作或事情可以同时发生，也可以先后发生。
① It was raining when we arrived. 当我们到达的时候，正下着雨。
② When we were at school, we went to the library every day. 我们在求学的时候，每天都到图书馆去。
Ⅲ. while 当(在)的时候表主句的动作或事情发生在从句中的动作或事情的进展过程之中。从句通常为进行时态。它有时可与when通用，但它只能指一段时间(a period of time),而不能指一点时间(a point of time)如上面的第一个例子。又如：
① Please dont talk so loud while others are working. 别人在工作的时候，请勿大声讲话。
② While I am washing the floor, you can be cleaning the windows.我洗地板时，你可以擦窗户。
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