Ⅰ. as “當(在)……時候”往往可與when 或while通用，但它著重指主句和從句的動作或事情相并發(fā)生。如：
① I saw him as he was getting off the bus. 當下公共汽車的時候，我看見了他。
② As he walked on, he felt himself getting more and more tired.他繼續(xù)往前走的時候，感到越來越疲乏。
Ⅱ. when “當(在)……的時候”主句和從句的動作或事情可以同時發(fā)生，也可以先后發(fā)生。
① It was raining when we arrived. 當我們到達的時候，正下著雨。
② When we were at school, we went to the library every day. 我們在求學的時候，每天都到圖書館去。
Ⅲ. while “當(在)……的時候”表主句的動作或事情發(fā)生在從句中的動作或事情的進展過程之中。從句通常為進行時態(tài)。它有時可與when通用，但它只能指一段時間(a period of time),而不能指一點時間(a point of time)如上面的第一個例子。又如：
① Please don’t talk so loud while others are working. 別人在工作的時候，請勿大聲講話。
② While I am washing the floor, you can be cleaning the windows.我洗地板時，你可以擦窗戶。
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