when,while,as,just as表示当的时候用法上有什么区别

1. 当从句中的谓语动词是持续性的动作时,我们可以用while, when或as。

While / When / As I was walking down the street, I noticed a police car in front of the bank.我顺着马路往前走时,发现银行门前停着一辆警车。

2. 当从句中的谓语动词表示瞬间动作时,不可用while。

He was on the point of leaving when someone knocked at the door. 他正要走,这时有人敲门。

3. 如果谈论两个长动作,最常用的是while。

While John was sitting biting his nails, I was working out a plan to get us home. 约翰坐在那里咬指甲时,我想出了一个回家的办法。

不过, as是可以用来说明两种正在发展或变化的情况的,这时as引导一个持续性的动作,主句中的动作与之同时进行。

As we talked on, he got more and more excited. 我们继续往下谈的时候,他越来越兴奋。

4. 如果表示两个短动作或事件同时发生,最常用的是as或just as,也可用when。

Just as he caught the ball, there was a tearing sound. 当他抓住球的时候,有一种撕裂的声音。

I thought of it just when you opened your mouth. 就在你要说话的时候,我也想到了。

at last / in the end / finally
三者都可作“最后；终于”解，但用法有所不同。

at last只能指时间位置，而不能指时间顺序，在语义上指经过，周折，等待，耽搁后的“最后，终于”得到所期待的结果。
1 At last , he passed the exam . 最后他终于通过了考试。（可用in the end代替）
2 His chance came at last. 他的机会终于来了。（强调他为获得此机会进行的各种努力。）
3 The war had been long and hard, but now there was peace at last.
经过漫长而艰苦的战争，现在和平终于到来了。

finally 指一系列事物或论点的顺序，在列举事物时可以用来引出最后一项内容 finally 在意义上与用法上与at last 有共同之处，但也有不同。finally 既可指时间位置（相当于at last和in the end），也可指时间的先后顺序。finally 有两个用法：一是在列举事物或论点时，可用来引出最后一项内容；二是用在句中动词前面，表示“等了好久才……”。

eg. Finally I’d like to thank you all for your coming . 最后我要感谢诸位的光临。（不能用at last）
2 They waited and waited , and the concert finally started. 他们等啊等啊，最后音乐会终于开始了。
3 Three policemen rode past on their motorbikes , then came the TV broadcasting car and finally the Marathon runners. 三个警察骑着摩托过去了，接着来了电视广播车，最后过来的是马拉松赛跑选手。

in the end 表示结局，有时可与at last 换用，不同的是in the end 也可以用于指将来时，其余两个则不行。
（1）All will come right in the end . 到头来一切都会好的。
（2）He tried many times to pass the examination , and in the end he succeeded. 他多次努力想通过考试，最后成功了。

when，while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只能和延续性动词连用。

a. 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？

b. Sorry，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。

c. Strike 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。

d. 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。

a. When 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。

b When 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。

a. When ／While 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。

b. When ／While 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．在将来时从句中，常用when，且从句须用一般时代替将来时。

a. You shall borrow the book when I have finished reading it．在我读完这本书后，你可以借阅。

b. When 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．．．

a. I 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。

b. I 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。

when , as , while 这三个词都可以引出时间状语从句，它们的差别是：when 从句表示某时刻或一段时间 as 从句表示进展过程，while 只表示一段时间。

When he left the house, I was sitting in the garden.

当他离开家时，我正在院子里坐着。

When he arrived home, it was just nine o’clock.

当他到家时，正是九点钟。

As he grew older, he became more intelligent.

随着他年龄的增长，他变得更有才智了。

While he was in London,he studied music.

当他在伦敦时，他学习音乐。

While she was typing the letter,the telephone rang.

当她在用打字机打那封信时，电话响了。