while和when和then的区别 如何区分when,while,then?
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①when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。
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。
than的难点用法
一、后接人称代词用主格还是宾格
由于than既可用作连词也可用作介词，所以当后接人称代词时，可用主格也可用宾格。通常认为在正式文体中多用主格，在口语或非正式文体中多用宾语。如：
Everyone here is taller than I [me]. 这儿的每一个人都比我高。
Her sister swims faster than she [her]. 她姐姐游泳比她快。
但是，若人称代词之后跟有动词，则只能用主格。如：
Everyone here is taller than I am. 这儿的每一个人都比我高。
Her sister swims faster than she does. 她姐姐游泳比她快。
注意：有时用主格或宾格会导致意思的变化。比较：
I love you more than he (likes you). 我比他更爱你。
I love you more than (he likes) him. 我爱你胜过爱他。
二、后接动词用不定式还是-ing分词
1. 当连接两个非谓语动词时，通常应使用一样的形式。如：
It is much easier to get into debt than to get out of it. 借债容易还债难。
He likes playing chess more than watching TV. 与看电视相比，他更喜欢下棋。
比较：
He thinks it is safer to drive himself than (to) let me drive.
=He thinks that driving himself is safer than letting me drive. 他认为他自己开车比让我开车要更安全些。
2. 若不是连接两个非谓语动词，则其后出现的动词通常用-ing分词形式(虽然也可用不定式，但不如用-ing分词普通)。如：
There are worse calamities than failing your driving test. 比起你驾驶考试不合格来说，更大的灾难还多着呢。
Nothing gives me more pleasure than listening to Mozart. 再没有比听莫扎特的乐曲更让我高兴的事了。
Nothing is more unpleasant than finding [to find] insects in your bath. 最使人不快的是在浴室里发现有虫子。
三、引导比较状语从句的时态问题
若than引导的比较状语从句与主句动作不一致，可以根据情况使用适当的时态形式。如：
He drives faster than he did a year ago. 他开车比一年以前快了。
The house is rather bigger than we thought. 这所房子比我们想的大得多。
若主句为将来时，than引导的从句可用现在时态表示将来，也可直接使用将来时态。如：
We’ll probably drive faster than you do [will]. 我们开车可能会比你们快。
四、引导比较状语从句的倒装问题
than引导比较状语从句时，从句语序通常不需倒装，但在正式文体中，有时也可倒装。如：
City dweller have a higher death rate than country people do.
=City dweller have a higher death rate than do country people. 城市居民死亡率比农村居民高。
五、引导比较状语从句且在从句中充当成分
有时可引导一个从句并在从句中充当句子成分(主语、宾语、表语)，为便于理解，有时可视为than后省略了what：
We don’t want to do more than is necessary. 我们不想做不必要的事情。
The pain was almost more than he could bear. 这样的痛苦几乎使他受不了。
She’s not a very good manager she always spends more money than she earns. 她不大会理财——总是入不敷出。
Life here is much easier than it used to be. 如今在此地生活比起从前可舒服多了。
比较下面一句，than后的expected为省略结构：
He returned three days earlier than (he was) expected. 他回来得比预料的要早三天。
六、后接时间或条件状语从句
若语义需要，有时其后可接when引导的时间状语从句以及if 引导的条件状语从句。如：
You are a little fatter than when I saw you last. 你比我上次见你时胖点了。
Joe carries only nine pounds more than when he was twenty. 乔比他20岁时只重了九磅。
They work better together than if they are alone. 他们一起干比他们单干效果要好。
七、同质比较还是异质比较
than 通常用于两个不同的人或物在同一方面进行比较，但有时指的可能是同一个人或物在两个不同的方面进行比较，此时只用于more…than…结构，而不能用 –er…than…的形式(即使是单音节也是如此)，这类结构通常可译为“与其……不如……”“有……无……”。如：
He’s more fat than short. 与其说他矮不如说他胖。
They are more brave than wise. 他们有勇无谋。
If we tell him about it, it may do more harm than good. 倘若我们把情况告诉他，那可能弊多利少。
八、与never (a) 连用时如何理解
有时than与带有never (a) 的比较级结构连用，表示强调，通常表示最高级的含义。如：
I’ve never had a worse morning than today. 我哪一天早上也不像今天早上这样倒霉。
若意思明确，有时可省略than结构。如：
He is fine, never better. 他很好，比以往任何时候都好。
I’ve never seen a finer bird. 我从未见过比这更美的鸟。
I’ve never found a better job. 这是我找到的最好的工作。
九、than any other后接名词用单数还是复数
than any other后接名词时通常用单数形式，很少用复数。如：
He swims faster than any other student in his class. 他比班上的其他任何学生都游得快。
More heat is lost through the head than any other part of the body. 从头部散发的热量比身体的其他任何部位都多。
十、几组容易理解出错的than习语
1.“no +比较级+than”与“not +比较级+than”
(1) “no +比较级+than”用于否定两者，表示“与……一样不”，其义大致相当于其中形容词或副词的反义词用于as…as…结构。如：
He is no richer than a beggar.=He is as poor as a beggar. 他穷如乞丐。
This one is no better than that one.=This one is as bad as that one. 这个与那个一样不好。
(2) “not +比较级+than”的意思是“不比……更”“不如”。如：
He is not richer than you. 他不如你富有。
This one is not better than that one. 这个不如那个好。
当前面主句中有位于动词，而相比较的对象，也就是than后面所接的对象恰好是动词的话就不能省略，但是如果比较的对象是sth时，就可以省略主句中的动词。如：The weather in Beijing is colder than that of Sichuan.
望采纳！~
as和when、while的区别
我英语很菜的!
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