有确定答案吗?我只晓得
一、
when 意为“当…时”,用作连词,引导时间状语从句,从句的谓语动词可以是延续的（此时相当于while）, 也可以是非延续性的.引导的时间状语从句通常用现在时表将来.
I will write to my sister when I have finished the book. 我读完这本书后便给姐姐写信.
When he sees the photo, he can't help thinking of his childhood.
每当看到这张照片,他就不禁想起童年.
二、
when 引导时间状语时,若主从句主语一致或从句主语为it且从句谓语动词含be的形式,可省略从句的主语及be 部分.
When (I was) walking on the street, I came across Mary. 我在街上漫步时遇上了玛丽.
He gave good practical advice when (he was) asked. 他有求必应,给出了好的可行性建议.
She would weep when (she was) alone. 她孤独时便哭.
When（you are） in trouble, you can ask her for help. 有麻烦时,你可找她帮忙.
We will do that only when ( it is) necessary. 只有当有必要时我们才会那么做.
三、
when用作连词,表突然发生某事,意为“就在这时/那时”（=at this/that time）,常构成句型 be doing… when… 或be about to do…/be on the point of doing… when….
I was just coming along to see you when I ran into Tom. 遇见汤姆时,我正在来看你的路上.
He was about to go to bed when the telephone rang. 他准备睡觉时电话铃响了.
四、
When用作连词表过早发生某事,意为“还没（刚刚）…就”,可构成句型 hardly/scarcely…when….使用该句型时应注意两点：① 主句的谓语动词用过去完成时,从句谓语动词用过去时.② 若hardly/scarcely 位于句首时,其后的句子部分倒装,而when引导的从句不倒装.
I had been there little more than a week when I set to work in earnest.
我在那还不到一个星期就开始认真工作.
Hardly had I opened the door when he hit me. 我一打开门他就撞了我.
六、
when引导条件状语从句,意为“在…情况下；如果；要是”,相当if.
Why do you walk when you have a car? 你有车为什么还走路呢?
No one can make a dress when they haven't learnt. 没有人能够不学就能制作衣服的.
七、
when 引导时间状语从句,意为“一…就 …”,相当于 as soon as .
We will start when everything is ready. 一切准备好了我们就出发.
When she comes, I will give it to her. 她来了我就给她.
八、
when可引导原因状语从句,意为“既然；考虑到…”,相当于since, as 或now that.
You can't go home when you haven't finished your work.
既然你没完成工作,你就不可以回家.
九、
when可引导名词性从句或接不定式.
Could you tell me when you will leave for Shanghai? 能告诉我你什么时候去上海吗?
Ask him when to open it. 告诉他什么时候打开.
十、
when作关系副词,引导定语从句,在从句中充当时间状语.
At the time when I saw him, he was very well. 我见到他时他很好.
Sunday is a holiday, when people do not go to work. 星期天是一个假日,这天人们不用去上班
