when是at or during the time that,既指时间点,也可指一段时间；
while是during the time
that,只指一段时间,因此when引导的时间状语从句中的动词可以是终止性动词,也可以是延续性动词,而while从句中的动词必须是延续性动词.
②when
说明从句的动作和主句的动作可以是同时,也可以是先后发生；while
则强调主句的动作在从句动作的发生的过程中或主从句两个动作同时发生.
③由when引导的时间状语从句,主句用过去进行时,从句应用一般过去时；如果从句和主句的动作同时发生,两句都用过去进行时的时候,多用while引导,如：
a.When
the teacher came in,we were talking.
当此句改变主从句的位置时,则为：
While we were
talking,the teacher came in.
b.They were singing while we were
dancing.
④when和while 还可作并列连词.when表示“在那时”；while表示“而,却”,表对照关系.如：
a.The
children were running to move the bag of rice when they heard the sound of a
motor bike.
孩子们正要跑过去搬开那袋米,这时他们听到了摩托车的声音.
b.He is strong while his brother
is weak.
他长得很结实,而他弟弟却很瘦弱.
具体你可以参考这一段.
when,while,as引导时间状语从句的区别
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、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 youve 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
phonecall,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,Ill talk with him about
this．下周,经理来这参观时,我会和他谈谈此事.
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