同学你好，该知识点来自沪江网校《BEC商务英语初、中、高级连读【签约 学霸班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学好，
这里助助总结下while，when和as的区别给同学参考哦。

英语中，when, while, as显然都可以引导时间状语从句，但用法区别非常大，一起来看一下：

一、when可以和延续性动词连用，也可以和短暂性动词连用；而while和as只能和延续性动词连用。

① Why do you want a new job when you’ve got such a good one already?
（get为短暂性动词）你已经找到如此好的工作，为何还想再找新的？  
②Sorry, I was out when you called me.
（call为短暂性动词）对不起，你打电话时我刚好外出了。
③Strike while the iron is hot.
（is为延续性动词，表示一种持续的状态）趁热打铁。  
④ The students took notes as they listened.
（listen为延续性动词）学生们边听课边做笔记。  

二、when从句的谓语动词可以在主句谓语动作之前、之后或同时发生；while和as从句的谓语动作必须是和主句谓语动作同时发生。  

1.从句动作在主句动作前/后发生，只用when。  
①When he had finished his homework, he took a short rest.
（finished先发生）当他完成作业后，他休息了一会儿。
②When I got to the airport, the guests had left.
（got to后发生）当我赶到飞机场时，客人们已经离开了。  

2.从句动作和主句动作同时发生，且从句动作为延续性动词时，when, while, as都可使用。  
①When ／While ／As we were dancing, a stranger came in.
（dance为延续性动词）当我们跳舞时，一位陌生人走了进来。  
②When ／While ／As she was making a phone call, I was writing a letter.
（make为延续性动词）当她在打电话时，我正在写信。  

3.当主句、从句动作同时进行，从句动作的时间概念淡化，而主要表示主句动作发生的背景或条件时，只能用as。这时，as常表示“随着……”；“一边……，一边……”之意。
① As the time went on, the weather got worse.（as表示“随着……”之意）  
② The atmosphere gets thinner and thinner as the height increases. 随着高度的增加，大气越来越稀薄。  
③As years go by, China is getting stronger and richer. 随着时间一年一年过去，中国变得越来越富强了。  
④The little girls sang as they went. 小姑娘们一边走，一边唱。  
⑤The sad mother sat on the roadside, shouting as she was crying. 伤心的妈妈坐在路边，边哭边叫。  

4.在将来时从句中，常用when，且从句须用一般时代替将来时。  
①You shall borrow the book when I have finished reading it. 在我读完这本书后，你可以借阅。  
②When the manager comes here for a visit next week, I‘ll talk with him about this.
下周，经理来这参观时，我会和他谈谈此事。  

三、when用于表示“一……就……”的句型中（指过去的事情）。  
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. 我刚一闭上眼，就有人在敲门了。  
②I had hardly ／scarcely entered my room when the telephone rang. ＝Hardly ／Scarcely had I entered my room when the telephone rang. 我刚一走进房门，电话就响了。


过去进行时表示在过去的某一时间正在进行的动作。如：
I was cooking when you called. 当你打电话的时候，我正在做饭。
He was having a meeting at 4 pm. yesterday. 昨天下午4点钟的时候，他正在开会。