when可以用作副词、连词和代词，其中副词可用作疑问副词、连接副词和关系副词等；连词可用作从属连词和并列连词等；用作代词位于介词之后。
when用作副词
用作疑问副词,引导特殊疑问句.例如：
When will you come to see me?你什么时候要来看我?
When are they going to visit the Great Wall?他们打算什么时候去游览长城?
用作连接副词,通常用来引导主语从句、表语从句、同位语从句、宾语从句及起名词作用的“ when ＋动词不定式”结构.例如：
When he comes is not known.他何时来还不知道.
The morning is when I am busiest.早上是我最忙的时候.
I have no idea when the game will begin.我不知道比赛何时开始.
I don't know when the plane takes off.我不知道飞机何时起飞.
I don't know when to leave for London.我不知道该在何时动身去伦敦.
用作关系副词,引导限定性定语从句和非限定性定语从句.例如：
Do you still remember the days when we stayed in America?你还记得我们呆在美国的那些日子吗?
The day will come soon when the Chinese astronauts will go to the moon.中国宇航员登上月球的那一天很快就要到来了.
It happened ten years ago,when I was a child.事情发生在十年前,当时我还是个小孩子.
We will go to the countryside at the beginning of June,when the summer harvest will start.我们将在六月初下乡,那时夏收就要开始了.
when用作连词
用作从属连词,意为“当……的时候”,引导时间状语从句.例如：
They learned a lot from the peasants when they stayed in the village.
他们住在那个村子时,从农民那里学到了许多东西.
It was snowing when he arrived at the station.他到达车站时,天正下着雪.
注意：如果 when 引导的从句中的主语和主句中的主语一致,且从句中的谓语动词是“ be ＋分词”或从句主语是 it ,则 be 动词及其主语常可省略.例如：
When（ he was ） asked why he was late,he made no answer.当被问到他为什么迟到时,他避而不答.
I'll tell him about it when （ it is ） possible.在可能的时候,我会把那件事告诉他的.
用作从属连词,意为“一……就……”,引导时间状语从句.例如：
We will stand up when the teacher comes into the classroom.老师一进教室我们就起立.
Fire the rockets when I give the signal.我一发信号,你就点燃火箭.
用作从属连词,意为“还没 / 刚刚……就”,引导时间状语从句.例如：
I had hardly opened the door when he came in.我刚一开门,他就进来了.
I had not been reading for half an hour when I heard someone call my name.我刚刚看了半个小时的书,就听到有人叫我的名字.
用作从属连词,意为“倘若,如果”,表示条件.例如：
He will be likely to recover when he is operated on.如果动手术,他有可能康复.
Turn off the switch when anything goes wrong with the machine.如果机器发生故障,就把电源关上.
用作从属连词,意为“既然,尽管”,表示让步.例如：
Why use metal when you can use plastic?既然能用塑料,为什么用金属呢?
They kept trying when they knew it was hopeless.尽管他们知道那件事没有希望,可是他们还在不断地努力.
用作并列连词,意为“在那时,届时”,表示时间.这时主句中可以用过去进行时,过去完成时或“ was/were about to do sth.”结构.例如：
Last night I was about to go to bed when the phone rang.昨天晚上我正要上床睡觉时,电话铃响了.
I was cooking in the kitchen when someone knocked at the door.我正在厨房做饭,这时有人敲门.
He had just finished the book when supper was served.我刚刚读完这本书,这时晚饭就端上来了.
用作并列连词,意为“虽然、然而、可是”,表示转折.例如：
He usually walks to work when he might take a bus.虽然他可以坐公共汽车上班,但他却常常步行上班.
I had only twenty dollars when I needed thirty to buy the dictionary.
我需要30美元买那本字典,可是我只有20美元.
用作并列连词,意为“而、却”,表示对比.例如：
How can he say that everything is fine when it's obvious that it is not?他怎能说一切都好呢?情况显然不是那样.
when用作代词
when作为代词常常位于介词之后,意为“那时,什么时候”.例如：
We came back on Tuesday,since when we have been working in the repair shop.我们星期二回来,从那时起就一直在修配车间工作.
Since when have you been studying Japanese?你是从何时起一直学习日语的?
另外,when 还可用作名词,前面常常用定冠词 the .the when 表示事件发生的时间,常常与 the where,the how 并列使用.例如：
He told the police the when and the how of the accident.他告诉警察事故发生的时间及发生的原委.
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