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 don’t 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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