when .While .as 在表示“当……的时候”,有什么区别,

while 后面接延续性行为,但这个动作肯定和主句动作时间不一样长.如：he was talking to me while he was watching the tv.应该说“看电视”比“谈话”时间长.而as通常两个动作一直相随,很可能时间相同.如：he was thinking of a question as watching the tv.这两个动作就是一直相随的.as意为“一边……一边……”或“与……同时”,重在表示动作同时发生、伴随进行.while只指“时间段”,不指“时间点”,从句的动词只限于持续性动词.1 从句表示"随时间推移"连词能用as,不用while.As the day went on,the weather got worse.日子一天天过去,天气越变越坏.2 as引导短暂性动作的动词.Just as I stopped my car,a man came up to me.———————————————————————————————— when,as,while用法之异同 请看99年高考的一个选择题：
－I′m going to the post office.
－_____ you are there,can you buy me some stamps?
A．As B．while C．Because D．If
根据本题语境,选项C、D显然不合题意,故答案应在A、B之间选择.为什么正确答案为B而不是A呢?

先看when,as,while在引导时间状语从句时的主要用法：

when表示“当……的时候”.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,ask her for help.
When（I was）sleeping,I never heard a thing.

as表示"当……时""一面……一面","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"一面……一面……".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 the wind rose the noise increased.
As it grew darker it became colder.
As he grew older he became more confident.

While表示"在……的时候"、"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：
Never get on or off a bus while it is in motion.
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加现在分词可代替状语从句.如：
We must strike while the iron is hot.
He took a bath while I was preparing dinner.
Come and pay a visit to the park while the flowers are in bloom.

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.


再看文章开头所给的高考题,主句动词为buy,从句动词为be,不是两个平行动作,显然不应选项A,