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when，while，as引導(dǎo)時間狀語從句的區(qū)別 

when和while的區(qū)別
①when是at or during the time that, 既指時間點(diǎn)，也可指一段時間；
while是during the time that，只指一段時間，因此when引導(dǎo)的時間狀語從句中的動詞可以是終止性動詞，也可以是延續(xù)性動詞，而while從句中的動詞必須是延續(xù)性動詞。
②when 說明從句的動作和主句的動作可以是同時，也可以是先后發(fā)生；while 則強(qiáng)調(diào)主句的動作在從句動作的發(fā)生的過程中或主從句兩個動作同時發(fā)生。
③由when引導(dǎo)的時間狀語從句，主句用過去進(jìn)行時，從句應(yīng)用一般過去時；如果從句和主句的動作同時發(fā)生，兩句都用過去進(jìn)行時的時候，多用while引導(dǎo)，如：
a. When the teacher came in, we were talking.
當(dāng)此句改變主從句的位置時，則為：
While we were talking, the teacher came in.
b. They were singing while we were dancing.
④when和while 還可作并列連詞。when表示“在那時”；while表示“而，卻”，表對照關(guān)系。如：
a. The children were running to move the bag of rice when they heard the sound of a motor bike.
孩子們正要跑過去搬開那袋米，這時他們聽到了摩托車的聲音。
b. He is strong while his brother is weak.
他長得很結(jié)實(shí)，而他弟弟卻很瘦弱。

when，while，as引導(dǎo)時間狀語從句的區(qū)別
when，while，as顯然都可以引導(dǎo)時間狀語從句，但用法區(qū)別非常大。
一、when可以和延續(xù)性動詞連用，也可以和短暫性動詞連用；而while和as只能和延續(xù)性動詞連用。
① Why do you want a new job when youve got such a good one already？（get為短暫性動詞）你已經(jīng)找到如此好的工作，為何還想再找新的？
②Sorry，I was out when you called me．（call為短暫性動詞）對不起，你打電話時我剛好外出了。
③Strike while the iron is hot．（is為延續(xù)性動詞，表示一種持續(xù)的狀態(tài)）趁熱打鐵。
④ The students took notes as they listened．（listen為延續(xù)性動詞）學(xué)生們邊聽課邊做筆記。
二、when從句的謂語動詞可以在主句謂語動作之前、之后或同時發(fā)生；while和as從句的謂語動作必須是和主句謂語動作同時發(fā)生。
1．從句動作在主句動作前發(fā)生，只用 when。
①When he had finished his homework，he took a short rest．（finished先發(fā)生）當(dāng)他完成作業(yè)后，他休息了一會兒。
②When I got to the airport，the guests had left．（got to后發(fā)生）當(dāng)我趕到飛機(jī)場時，客人們已經(jīng)離開了。
2．從句動作和主句動作同時發(fā)生，且從句動作為延續(xù)性動詞時，when，while，as都可使用。
①When ／While ／As we were dancing，a stranger came in．（dance為延續(xù)性動詞）當(dāng)我們跳舞時，一位陌生人走了進(jìn)來。
②When ／While ／As she was making a phonecall，I was writing a letter．（make為延續(xù)性動詞）當(dāng)她在打電話時，我正在寫信。
3．當(dāng)主句、從句動作同時進(jìn)行，從句動作的時間概念淡化，而主要表示主句動作發(fā)生的背景或條件時，只能用 as。這時，as常表示“隨著……”；“一邊……，一邊……”之意。
① As the time went on，the weather got worse．（as表示“隨著……”之意）
② The atmosphere gets thinner and thinner as the height increases．隨著高度的增加，大氣越來越稀薄。
③As 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。
④The little girls sang as they went．小姑娘們一邊走，一邊唱。
⑤The sad mother sat on the roadside，shouting as she was crying．傷心的媽媽坐在路邊，邊哭邊叫。
4．在將來時從句中，常用when，且從句須用一般時代替將來時。
①You shall borrow the book when I have finished reading it．在我讀完這本書后，你可以借閱。
②When 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?
三、when用于表示“一……就……”的句型中（指過去的事情）。
sb．had hardly（＝scarcely） done sth．when．．．＝Hardly ／ Scarcely had sb．done sth．when．．．
①I had hardly ／scarcely closed my eyes when someone knocked at the door．＝Hardly ／ Scarcely had I closed my eyes when someone knocked at the door．我剛一閉上眼，就有人在敲門了。
②I had hardly ／scarcely entered my room when the telephone rang．＝Hardly ／Scarcely had I entered my room when the telephone rang．我剛一走進(jìn)房門，電話就響了。