when, while, as引导时间状语从句时的区别

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     when, while, as三者都可表示“当……时候”,但它们在用法上有区别:

一、若主句表示的是一个短暂性动作,从句表示的是一个持续性动作,三者都可用:
     He fell asleep when [while, as] he was reading. 他看书时睡着了。

二、若主、从句表示两个同时进行的持续性动作,且强调主句表示的动作延续到从句所指的整个时间,通常要用 while:
     Don’t talk while you’re eating. 吃饭时不要说话。
     I kept silent while he was writing. 在他写的时候,我默不作声。

三、when表示“当……时候”。从句中既可用延续性动词,又可用非延续性动词,这些动词既可以表示动作,又可表示状态。从句中的动作既可和主句的动作同时发生,又可在主句的动作之前或之后发生。如:
     When I came into the room, I found him lying there asleep.
     Mary was having dinner when I saw her.
     When you meet a word you don't know, consult the dictionary.
     She was beautiful when she was a girl.

     如果when从句的主语与主句主语相同,谓语又是be动词,从句主语和be可以省略:当when从句主语与主句主语相同时,用when加分词可以替代状语从句。如:
     When(you are)in trouble, you can ask her for help.
     When(I was)sleeping, I never heard a thing.

四、While表示"在……时候"、"在……期间"。它强调主句的动作与从句的动作同时发生或主句的动作发生在从句的动作过程中。While从句中必须是表示动作或状态的延续性动词。如:
     The weather was fine while we were in Beijing.
     She called while I was out.

     1、如果主句的动作在从句的动作过程中发生,从句常用进行时。如:
     While we were swimming someone stole our clothes.
     Don′t talk so loud while others are working.

     2、如果主句和从句主语相同,while加现在分词可替代状语从句。如:
     While she was walking along the street,she was hit by a car.(替代前)
     While walking along the street,she was hit by a car.(替代后)

五、as表示"当……时","一边……一边","随着"。其具体用法如下:
     1、表示"当……时"、"和……同时"。常指从句的动作未结束,主句中的动作就已发生。从句中多用表示动作的动词,而不用be动词或表示感觉、理解、知道这类动词。
     As he stood there, he saw two men enter the bar.
     She dropped the glass as she stood up.
     As I was walking down the street, an American asked me for directions to the nearest station.

     2、用于平行的动作中,表示"一边……一边"。常指一个主语同时进行两个动作。如:
     He sang as he worked.
     The students took notes as they listened.

     3、用于平行发展的结构中,表示"随着……"。常指一个行为是另一个行为的结果,或一种状态随另一种状态变化。句中的动词多表示状态的发展变化。
     As it grew darker it became colder.
     As he grew older he became more confident.

     通过以上分析我们可以看出,when, as, while引导时间状语从句时,侧重点有所不同。
     1、When可指时间点,又可指时间段,从句中可用延续性动词,也可用非延续性动词。
     2、While总是指一个时间段,从句中必须用延续性动词。
     3、as多用于平行结构。表示两个动作同时发生:一个主语同时进行两个动作:一种状态随另一种状态的变化而变化。
     4、表示主句的动作发生在从句的较长动作过程中,when, while, as均可使用。从句中为be动词时,多用when和while。谈论两个同时进行的长动作可用while;谈论两个平行动作或两种状态变化多用as。