when，while，as引導(dǎo)時(shí)間狀語從句的區(qū)別
when，while，as顯然都可以引導(dǎo)時(shí)間狀語從句，但用法區(qū)別非常大。 
一、when可以和延續(xù)性動(dòng)詞連用，也可以和短暫性動(dòng)詞連用；而while和as只能和延續(xù)性動(dòng)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(dòng)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(dòng)詞）對(duì)不起，你打電話時(shí)我剛好外出了。 
③Strike while the iron is hot．（is為延續(xù)性動(dòng)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(dòng)詞）學(xué)生們邊聽課邊做筆記。 

二、when從句的謂語動(dòng)詞可以在主句謂語動(dòng)作之前、之后或同時(shí)發(fā)生；while和as從句的謂語動(dòng)作必須是和主句謂語動(dòng)作同時(shí)發(fā)生。 
1．從句動(dòng)作在主句動(dòng)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(huì)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(chǎng)時(shí)，客人們已經(jīng)離開了。 
2．從句動(dòng)作和主句動(dòng)作同時(shí)發(fā)生，且從句動(dòng)作為延續(xù)性動(dòng)詞時(shí)，when，while，as都可使用。 
①When ／While ／As we were dancing，a stranger came in．（dance為延續(xù)性動(dòng)詞）當(dāng)我們跳舞時(shí)，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(dòng)詞）當(dāng)她在打電話時(shí)，我正在寫信。 
3．當(dāng)主句、從句動(dòng)作同時(shí)進(jìn)行，從句動(dòng)作的時(shí)間概念淡化，而主要表示主句動(dòng)作發(fā)生的背景或條件時(shí)，只能用 as。這時(shí)，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(shí)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(shí)從句中，常用when，且從句須用一般時(shí)代替將來時(shí)。 
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(shí)，我會(huì)和他談?wù)劥耸隆?nbsp;

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。

  ④當(dāng)從句的謂語動(dòng)詞是終止性動(dòng)詞，而且主句的謂語動(dòng)詞也是終止性動(dòng)詞時(shí),when可和as通用，而且用as比用when在時(shí)間上更為緊湊，有"正當(dāng)這時(shí)"的含義。如：

  He came just as (or when) I reached the door.我剛到門那兒，他就來了。

  ⑤從句的謂語動(dòng)詞如表示狀態(tài)時(shí)，通常用while。如：

  We must strike while the iron is hot.我們應(yīng)該趁熱打鐵。

  ⑥while和when都可以用作并列連詞。

  Mike is tall while his brother is short.邁克的個(gè)子很高，然而他弟弟很矮。

  Mary was on her way home when two big boys stopped her.瑪麗正走在回家的路上，這時(shí)兩個(gè)大個(gè)子男孩攔住她。