while 后面接延續(xù)性行為,但這個(gè)動(dòng)作肯定和主句動(dòng)作時(shí)間不一樣長(zhǎng).如：he was talking to me while he was watching the tv.應(yīng)該說(shuō)“看電視”比“談話”時(shí)間長(zhǎng).而as通常兩個(gè)動(dòng)作一直相隨,很可能時(shí)間相同.如：he was thinking of a question as watching the tv.這兩個(gè)動(dòng)作就是一直相隨的.as意為“一邊……一邊……”或“與……同時(shí)”,重在表示動(dòng)作同時(shí)發(fā)生、伴隨進(jìn)行.while只指“時(shí)間段”,不指“時(shí)間點(diǎn)”,從句的動(dòng)詞只限于持續(xù)性動(dòng)詞.1 從句表示"隨時(shí)間推移"連詞能用as,不用while.As the day went on,the weather got worse.日子一天天過(guò)去,天氣越變?cè)綁?2 as引導(dǎo)短暫性動(dòng)作的動(dòng)詞.Just as I stopped my car,a man came up to me.———————————————————————————————— when,as,while用法之異同 請(qǐng)看99年高考的一個(gè)選擇題：
－I′m going to the post office.
－_____ you are there,can you buy me some stamps?
A．As B．while C．Because D．If
根據(jù)本題語(yǔ)境,選項(xiàng)C、D顯然不合題意,故答案應(yīng)在A、B之間選擇.為什么正確答案為B而不是A呢?

先看when,as,while在引導(dǎo)時(shí)間狀語(yǔ)從句時(shí)的主要用法：

when表示“當(dāng)……的時(shí)候”.從句中既可用延續(xù)性動(dòng)詞,又可用非延續(xù)性動(dòng)詞,這些動(dòng)詞既可以表示動(dòng)作,又可表示狀態(tài).從句中的動(dòng)作既可和主句的動(dòng)作同時(shí)發(fā)生,又可在主句的動(dòng)作之前或之后發(fā)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從句的主語(yǔ)與主句主語(yǔ)相同,謂語(yǔ)又是be動(dòng)詞,從句主語(yǔ)和be可以省略：當(dāng)when從句主語(yǔ)與主句主語(yǔ)相同時(shí),用when加分詞可以替代狀語(yǔ)從句.如：
When（you are）in trouble,ask her for help.
When（I was）sleeping,I never heard a thing.

as表示"當(dāng)……時(shí)""一面……一面","隨著".其具體用法如下：

1、表示"當(dāng)……時(shí)"、"和……同時(shí)".常指從句的動(dòng)作未結(jié)束,主句中的動(dòng)作就已發(fā)生.從句中多用表示動(dòng)作的動(dòng)詞,而不用be動(dòng)詞或表示感覺(jué)、理解、知道這類動(dòng)詞.
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、用于平行的動(dòng)作中,表示"一面……一面……".常指一個(gè)主語(yǔ)同時(shí)進(jìn)行兩個(gè)動(dòng)作.如：
He sang as he worked.
The students took notes as they listened.

3、用于平行發(fā)展的結(jié)構(gòu)中,表示"隨著……".常指一個(gè)行為是另一個(gè)行為的結(jié)果,或一種狀態(tài)隨另一種狀態(tài)變化.句中的動(dòng)詞多表示狀態(tài)的發(fā)展變化.
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……的時(shí)候"、"在……期間".它強(qiáng)調(diào)主句的動(dòng)作與從句的動(dòng)作同時(shí)發(fā)生或主句的動(dòng)作發(fā)生在從句的動(dòng)作過(guò)程中.While從句中必須是表示動(dòng)作或狀態(tài)的延續(xù)性動(dòng)詞.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、如果主句的動(dòng)作在從句的動(dòng)作過(guò)程中發(fā)生,從句常用進(jìn)行時(shí).如：
While we were swimming someone stole our clothes.
Don′t talk so loud while others are working.

2、如果主句和從句主語(yǔ)相同,while加現(xiàn)在分詞可代替狀語(yǔ)從句.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.

通過(guò)以上分析我們可以看出,when,as,while引導(dǎo)時(shí)間狀語(yǔ)從句時(shí),側(cè)重點(diǎn)有所不同.
1、When可指時(shí)間點(diǎn),又可指時(shí)間段,從句中可用延續(xù)性動(dòng)詞,也可用非延續(xù)性動(dòng)詞.
2、While總是指一個(gè)時(shí)間段,從句中必須用延續(xù)性動(dòng)詞.
3、as多用于平行結(jié)構(gòu).表示兩個(gè)動(dòng)作同時(shí)發(fā)生：一個(gè)主語(yǔ)同時(shí)進(jìn)行兩個(gè)動(dòng)作：一種狀態(tài)隨另一種狀態(tài)的變化而變化.
4、表示主句的動(dòng)作發(fā)生在從句的較長(zhǎng)動(dòng)作過(guò)程中,when,while,as均可使用.從句中為be動(dòng)詞時(shí),多用when和while.談?wù)搩蓚€(gè)同時(shí)進(jìn)行的長(zhǎng)動(dòng)作可用while；談?wù)搩蓚€(gè)平行動(dòng)作或兩種狀態(tài)變化多用as.


再看文章開頭所給的高考題,主句動(dòng)詞為buy,從句動(dòng)詞為be,不是兩個(gè)平行動(dòng)作,顯然不應(yīng)選項(xiàng)A,