1. 表示"当……的时候",相当于at the moment when。例如:
When we got to the cinema, the film had been on for half an hour. 当我们到达电影院时,电影已放映半小时了。 She was writing a letter when I came in. 当我进来时,她在写信。
2. 表示"一……就……",相当于as soon as。例如: The students got up when the bell rang. 铃声一响,学生们就起床了。 I'll ring you up when I arrive in Beijing. 我一到北京就给你打电话。
3. 表示"就在这时;当时",相当于just at the moment或just then .例如: We were about to start out when it began to rain heavily. 我们正要启程,就在这时,天下起了大雨。
He had just returned from one business trip when he was asked to make another one. 他刚刚出差回来,这时,又叫他再次出差。
4. 表示"每当;每次",相当于every time或whenever。例如: She always turns to us for help when
she is in trouble.每当她遇到困难,她总是向我们求助。 It is freezing cold here when it snows. 每当这儿下雪,天气就十分寒冷。 5. 表示"当……之后;在……以后",相当于after the time that。例如: We went home when the film was over. 电影结束以后,我们回家去了。 When she got home, she started to prepare supper. 她回到家后,开始准备晚饭。
二、引导条件状语从句
when引导的条件状语从句相当于if/ in case引导的条件状语从句.例如:
When there is no gravity, our feet can no longer stay on the ground. 如果没有重力,我们的脚就不能够在地面上站稳。Turn off the switch when anything goes wrong with the machine. 如果机器发生故障,就把电闸关上。
三、引导原因状语从句 when引导原因状语从句时,相当于since/now that引导的原因状语从句。 例如: How can they learn anything when they spend all their spare time watching television? 他们把所有的空闲时间都用来看电视了,还能学到什么东西呢? Why do you want a new job when you have got such a good one? 既然你已经有了这么好的一份工作,为什么还要再新的工作呢?
四、引导让步状语从句 when引导的让步状语从句相当于though或although引导的让步状语从句。例如: He usually walks when he might ride. 虽然有车可乘,但他通常是步行。
The little girl can tell right from wrong when she is only twelve. 这个小女孩虽然只有十二岁,却能分清是非。
五、引导定语从句 when引导定语从句时,有时可用in which或on which来替代。 例如: We'll put off the picnic until next week, when the weather may be better. 我们要把野餐推迟到下周,那时可能天气好些。
I'll never forget the day when I joined the Party. 我将永远不会忘记我入党的那一天。
六、引导名词性从句 1. 引导宾语从句。 例如: Please tell us when his father will return from abroad. 请告诉我们他父亲什么时候从国外回来。 2. 引导主语从句。例如:
When they will leave for Australia hasn't been decided. 他们什么时候动身去澳大利亚还没有决定。
3. 引导表语从句。例如: The question is when they will get so much money to set up the factory. 问题是他们将在什么时候弄到这么多钱来把这个工厂办起来。
when, while 和 as 引导时间状语从句的用法
这三个词的意思很简单，都有“当……时候”的意思。但学生经常会问三个词的区别在哪儿，特别是在做选择题的时候。别说是学生，就我个人而言，做这样的选择题要保证百分之百的正确也是不可能的。现根据大量的实例和个人的思考，做一点小结，供大家参考。
一、when 的用法 如果只从现象来看，when 从句用的最多的是一般过去时，而主句的时态没有限制,根据具体情况而定1. When he was a child he was always trying out new ideas. 他小时候就常常试验一些新的设想。 2. When she came into my room, I was just reading a book. 她走进我房间时，我正在看书。
3. Were you writing when the teacher came in? 老师进来的时候，你在写信吗？
用动词写一段话 4. Sorry, I was out when you called me. 对不起，你打电话来的时候我出去了。 5. He was on the point of leaving when someone knocked at the door. 他正要走，这时有人敲门。
6. I thought of it just when you opened your mouth. 就在你要说话的时候，我也想到了。
7. I had hardly[scarcely] closed my eyes when someone knocked at the door.
我刚一闭上眼，就有人在敲门了。
根据以上的例句，我们可以总结出一点：when 从句的A事件，相当于另一个事件B发生的时间点。也就是说，when 从句的重点不在动作本身发生的状态，而只是把它作为一个时间点，所以when 多数情况下用的是一般过去时，则不用正在进行时。因为如果用正在进行时，它表示的就是一段时间而不是一个时间点了。根据这一点，有的文章补充说：when 从句的动词大多是瞬时动词。这种说法也可以参照。
实际上，when 从句也可以有其它的时态，但几乎也不用进行时，因为它也只是作为一个时间参照点。例如：1. When I got to the airport, the guests had left. 当我赶到飞机场时，客人们已经离开了。
2. When he had finished his homework, he took a short rest.
当他完成作业后，他休息了一会儿。
3. Why do you want a new job when you have got such a good one already?
你已经到如此好的工作，为何还想再新的？
4. You shall borrow the book when I have finished reading it.
在我读完这本书后，你可以借阅。
5. When 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。
二、while 的用法
相比于when 来说，while 从句的侧重点就不一样了。while 从句的侧重点在于描述动作正在发生的状态，它的意思是：当while 事件正在发生的时候，另一件事如何如何。所以，while 从句一般用的是正在进行时。而另一件事的状态没有硬性的要求，根据具体情况而定。例如：1. While my wife was reading the newspaper, I was watching TV.
当妻子正在看报纸的时候，我正在看电视。
2. While Jim was mending his bike, Lin Tao came to see him.
正当吉姆修自行车时，林涛来看他。
3. While they were talking, the bell rang. 正在他们谈话的时候，上课铃响了。
4. You can’t do your homework while you’re watching TV.
你不能一边看电视一边做家庭作业。
5. 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，我正在制定一个回家的计划。
从时间的角度来看，while 表示的是一段时间，是一个过程。这是while 的侧重点。因此，如果含有“一段时间”的含义的时候，就可以用while。
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