when、while、as引導(dǎo)時間狀語從句的區(qū)別
一、when 的用法
如果只從現(xiàn)象來看，when 從句用的最多的是一般過去時，而主句的時態(tài)沒有限制，根據(jù)具體情況而定。
When he was a child he was always trying out new ideas.
他小時候就常常試驗一些新的設(shè)想。
when she came into my room I was just reading a book.
她走進(jìn)我房間時，我正在看書。
Were you writing when the teacher came in?
老師進(jìn)來的時候，你在寫信嗎?
Sorry，I was out when you called me.
對不起，你打電話來的時候我出去了。
He was on the point of leaving when someone knocked at the door.
他正要走,這時有人敲門。
I thought of it just when you opened your mouth.
就在你要說話的時候,我也想到了。
I had hardly[scarcely] closed my eyes when someone knocked at the door.
我剛一閉上眼，就有人在敲門了。
根據(jù)以上的例句，我們可以總結(jié)出一點：when 從句的A事件，相當(dāng)于另一個事件B發(fā)生的時間點。也就是說，when 從句的重點不在動作本身發(fā)生的狀態(tài)，而只是把它作為一個時間點，所以when 多數(shù)情況下用的是一般過去時，則不用正在進(jìn)行時。因為如果用正在進(jìn)行時，它表示的就是一段時間而不是一個時間點了。根據(jù)這一點，有的文章補(bǔ)充說：when 從句的動詞大多是瞬時動詞。這種說法也可以參照。
實際上，when 從句也可以有其它的時態(tài)，但幾乎也不用進(jìn)行時，因為它也只是作為一個時間參照點。例如：
When I got to the airport，the guests had left.
當(dāng)我趕到飛機(jī)場時，客人們已經(jīng)離開了。
When he had finished his homework，he took a short rest.
當(dāng)他完成作業(yè)后，他休息了一會兒。
Why do you want a new job when you have got such a good one already?
你已經(jīng)找到如此好的工作，為何還想再找新的?
You shall borrow the book when I have finished reading it.
在我讀完這本書后，你可以借閱。
When the manager comes here for a visit next week，Ill talk with him about this. 下周，經(jīng)理來這參觀時，我會和他談?wù)劥耸隆?/p>
二、while 的用法
相比于when 來說，while 從句的側(cè)重點就不一樣了。while 從句的側(cè)重點在于描述動作正在發(fā)生的狀態(tài)，它的意思是：當(dāng)while 事件正在發(fā)生的時候，另一件事如何如何。所以，while 從句一般用的是正在進(jìn)行時。而另一件事的狀態(tài)沒有硬性的要求，根據(jù)具體情況而定。例如：
While my wife was reading the newspaper, I was watching TV.
當(dāng)妻子正在看報紙的時候，我正在看電視。
While Jim was mending his bike, Lin Tao came to see him.
正當(dāng)吉姆修自行車時，林濤來看他。
While they were talking , the bell rang.
正在他們談話的時候，上課鈴響了。
You can't do your homework while you're watching TV.
你不能一邊看電視一邊做家庭作業(yè)。
While John was sitting biting his nails, I was working out a plan to get us home. 約翰坐在那里咬指甲時,我想出了一個回家的辦法。
從時間的角度來看，while 表示的是一段時間，是一個過程。這是while 的側(cè)重點。因此，如果含有“一段時間”的含義的時候，就可以用while。例如：
Strike while the iron is hot. 趁熱打鐵。
這句話中，是說趁著鐵是熱的這段時間，趕緊打鐵。如果換成 when 意思就變了，相當(dāng)于說鐵只熱了一下，打一下，然后鐵就冷了。這顯然不符合文意。再例：
—I'm going to the post office.
—While you're there, can you get me some stamps?
三、as 的用法
1、as 從句表示的也是一件事情正在發(fā)生，另一件事也正在進(jìn)行當(dāng)中。但與 while 從句不同的是，as 從句用的一般不用正在進(jìn)行時，而只是一般過去時。as 從句一般可以翻譯成“邊……邊……”。例如：
As my mother sang those old songs, tears ran down her cheeks.
當(dāng)我媽媽唱起那些老歌時，眼淚順著她的臉頰流了下來。
The students took notes as they listened.
學(xué)生們邊聽課邊做筆記。
As we talked on, he got more and more excited.
我們繼續(xù)往下談的時候,他越來越興奮。
The little girls sang as they went.
小姑娘們一邊走，一邊唱。
Just as he caught the ball, there was a tearing sound.
當(dāng)他抓住球的時候,有一種撕裂的聲音。
2、as 表達(dá)的事件，往往只是主句動作發(fā)生的背景或條件時，as 只是一個次要的時間說明，不像while 從句有強(qiáng)調(diào) while 動作本身的意思。因此，as 常常翻譯成“隨著……”之意。 例如：
As the time went on，the weather got worse.
隨著時間的推移，氣候更加糟糕。
The atmosphere gets thinner and thinner as the height increases.
隨著高度的增加，大氣越來越稀薄。
As years go by，China is getting stronger and richer.
隨著時間一年一年過去，中國變得越來越富強(qiáng)了。
3、少數(shù)情況下，如果強(qiáng)調(diào)動作正在發(fā)生，as 從句也可以用正在進(jìn)行時。這只能算是特例了。
The sad mother sat on the roadside，shouting as she was crying.傷心的媽媽坐在路邊，邊哭邊叫。
As we was going out, it began to snow.
我們出門時，開始下雪了。
He came in as I was going to bed.
正要上床睡覺，他進(jìn)來了。
四、when, while, as 的互換
如果從句動作和主句動作同時發(fā)生，并且從句動作為延續(xù)性動詞時，when，while，as 可以互換使用。這種情況下，它們的細(xì)微區(qū)別恐怕連英、美人自己也說不清了。
When /While /As we were dancing，a stranger came in.
當(dāng)我們跳舞時，一位陌生人走了進(jìn)來。 [dance 為延續(xù)性動詞]
When /While /As she was making a phonecall，I was writing a letter.
當(dāng)她在打電話時，我正在寫信。 [make為延續(xù)性動詞]
While/When/ As I was walking down the street, I noticed a police car in front of the bank.
我順著馬路往前走時,發(fā)現(xiàn)銀行門前停著一輛警車。
[附] 常用的表示時間的關(guān)聯(lián)詞
常用關(guān)聯(lián)詞：when, as, while, as soon as, while, before, after, since , once,,till, until 等。
特殊關(guān)聯(lián)詞：the minute, the moment, the second, every time, the day，the instant, immediately , directly, instantly, no sooner … than, hardly …when, scarcely … when 等。
例句：
I didn't realize how special my mother was until I became an adult.
The children ran away from the orchard the moment they saw the guard.
No sooner had I arrived home than it began to rain.
Every time I listen to your advice, I get into trouble.
