一 .when 用作副词.

　　1.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?

　　2.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.

　　3.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.

　　二 .when 用作连词.

　　1.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.

　　2.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.