as、when和while的區(qū)別

　　Ⅰ. as 當(dāng)(在)時(shí)候往往可與when 或while通用，但它著重指主句和從句的動(dòng)作或事情相并發(fā)生。如：

　　① I saw him as he was getting off the bus. 當(dāng)下公共汽車的時(shí)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(shí)候，感到越來越疲乏。

　　Ⅱ. when 當(dāng)(在)的時(shí)候主句和從句的動(dòng)作或事情可以同時(shí)發(fā)生，也可以先后發(fā)生。

　　① It was raining when we arrived. 當(dāng)我們到達(dá)的時(shí)候，正下著雨。

　　② When we were at school, we went to the library every day. 我們?cè)谇髮W(xué)的時(shí)候，每天都到圖書館去。

　　Ⅲ. while 當(dāng)(在)的時(shí)候表主句的動(dòng)作或事情發(fā)生在從句中的動(dòng)作或事情的進(jìn)展過程之中。從句通常為進(jìn)行時(shí)態(tài)。它有時(shí)可與when通用，但它只能指一段時(shí)間(a period of time),而不能指一點(diǎn)時(shí)間(a point of time)如上面的第一個(gè)例子。又如：

　　① Please dont talk so loud while others are working. 別人在工作的時(shí)候，請(qǐng)勿大聲講話。

　　② While I am washing the floor, you can be cleaning the windows.我洗地板時(shí)，你可以擦窗戶。