一、作为副词，它有以下的用法:
1、作为疑问副词，引导特殊疑问句,意为“什么时候；何时”。如:
When will they come back?
他们什么时候回来？
When did you last see him?
你上次什么时候见到他的？
When can I see you?
我什么时候可以见你？
When would such a solution be possible?
什么情况下可以这么解决？
What time will they come back?
他们什么时候回来？
回答 when 引导的从句，不一定指出具体的时间点。如回答句 when 可用 tomorrow, next month 等。而回答 what time 引导的问句，则必须说出具全的时间点，如 at two o'clock, at five past ten 等。
2、作连接副词，引导名词性从句或不定式，意为“什么时候”。从句使用陈述句语序，时态根据实际情况而定。如:
主语从句：
When she'll be back depends much on the weather.
她何时回来在很大程度上取决于天气。
When he was born is unknown.
他生于何时还不知道。
When we can begin the expedition is still a question.
我们什么时候开始探险仍然是个问题。
宾语从句：
I'd like to know when they will come.
我想知道他们什么时候来。
I don't know when the decision was made.
我不知道决定是什么时候作出的。
I don't care when you clean the house. it's up to you.
我才不管你什么时候打扫房子呢，这由你自己决定。
Can you tell me when the bank opens?
你能告诉我银行什么时候开门吗?
不定式结构：
Have you decided when to go sightseeing?
你们已经决定什么时候去郊游了吗?
You should know when to stop asking questions.
你应该知道什么事不该问。
You have to know what's what and when to draw the line.
你必须搞清状况以及自己的底线。
3、作疑问代词，常和介词 since，till，up to 等连用，意为“什么时候”。如:
since:
Since when have you been interested in poetry?
你从什么时候开始对诗歌感兴趣的？
Since when have they had the house?
他们从什么时候起有这幢房子的?
Since when have you been escaping the reality?
你是从何时开始逃避现实的？
till:
Till when is the library open?
图书馆开放到几点呢?
Till when should I wait for him?
我要等他到什么时候？
Till when will I pay?
我何时才可以付钱？
4、作关系副词，引导限制性定语从句和非限制性定语从句。如:
限制性定语从句：
There are occasions when (on which) one must yield.
任何人都有不得不屈服的时候。
I will never forget the days when (in which) we shared all the roses and thorns.
我永远都不会忘记我们在一起同甘共苦的日子。
Do you remember the day when (in which) the three of us went on a picnic?
你还记得我们三个人去野餐的那一天吗?
非限制性定语从句：
It was on Mid-Autumn Festival, when all family members reunite, that I left my hometown for a strange city alone.
正是在中秋节这个家人团圆的日子，我却独自离开家乡去了一个陌生的城市。
He will put off the picnic until May 1st, when he will be free.
他将把野餐推迟到5月1日，那时他有空。
在定语从句中, that 在某些条件下可用来代替when ,这时其先行词通常是 time, day, moment, year 等。如：
His mother died on the evening that (when / on which)he was born.
他母亲死于他出生的那晚。
His father died the year that (when / on which) he was born.
他父亲在他出生那年逝世了。
He is unlikely to find the place that(where / in which) he lived forty years ago.
他不大可能找到他四十年前住的地方。
I stay at home on the days that I am not busy.
在我不忙的时候，我就呆在家里。
在某些含有“时间”的名词词组后面，由于该名词不表示时间，或整个名词词组已用作连词，所以不用 when 引导。如: the moment, the instant, the minute, the second 等等，意为“一……就……”，引导时间状语从句。如:
the moment:
The moment I closed my eyes, I fell asleep.
我闭上眼睛就睡着了。
I remember now. He arrived just at the moment it happened.
现在我想起来了，他恰恰在事情发生的时候到了。
the instant:
He'll telephone me the instant he knows.
他一知道，会即刻电告我的。
The instant I saw him I knew he was my brother.
我一看到他就知道他是我兄弟。
the minute:
The minute you do this, you'll lose control.
你一旦做了这件事，就会失去控制。
I knew it the minute I saw him.
我一看见他马上就知道了。
the first time:
The first time I went to China, I visited Beijing.
我第一次来中国就游览了北京。
I thought her nice and honest the first time I met her.
我第一次见到她时就认为她很诚实。
二、作为从属连词，引导状语从句，表示多种语法意义:
1、表示时间，意为“当……时；在……的时候”。
A、在时间、条件等状语从句里，用一般现在时表示将来时；当需要表达将来完成的意义时，必须使用现在完成时来代替。如:
一般现在时表将来：
When Bill comes, ask him to wait for me.
比尔来时，请他等我。
When you see him, please say hello to him.
见到他时,代我问他好。
现在完成时表将来：
When you have got a handle on your anxiety you can begin to control it.
一旦了解了自己焦虑的原因，你就开始能控制它了。
When you have finished your experiment, please tidy the lab and put everything back in the cupboards.
当你们做完实验后,请把实验室整理好并把物品放回柜子中。
B、表示过去发生的事情，在 when 引导的时间状语从句的主从复合句中，动作发生在先的用过去完成时，在后的用过去时。如：
When I was in Japan, I bought some beautiful pearls.
我在日本时，买了一些漂亮的珍珠。
I was a little surprised when she consented.
我有点儿吃惊她竟然同意了。
When he returned, she and Simon had already gone.
他回来时，她和西蒙已经走了。
When I reached the station, the train had left.
当我到达火车站时，火车已经开走了。
2、表示前面的叙述所没有提到过的信息。它总是对所描述的事件予以引人注目的强调，意为“这时；突然”；当位于句末的分句具有引种用法时，大致有以下三种情况：
A、when 分句前面的分句使用过去进行时。如:
He was still smiling when the door opened and his wife came in.
他正笑着的时候，门突然开了，他妻子走了进来。
I was thinking of this when I heard my name called.
我正想着这件事时，突然听到有人叫我的名字。
B、when 分句前面的分句使用 was(were) about to, was (were) on the point 等。如:
We were about to start when it began to rain.
我们刚要出发就开始下雨了。
He was on the point of leaving when someone knocked at the door.
他刚要走就有人敲门。
C、when 分句之前的分句用过去完成时,其中 sb had hardly / scarcely /barely … when 已成为固定词组。如:
We had hardly fallen asleep when the bell rang.
我们刚刚入睡，铃声就响了起来。
He had scarcely arrived when he had to leave again.
他刚刚到达就又要离开了。
3、表示条件，相当于 if ，引导条件状语从句。 如：
How can I get a job when I can't even read or write?
如果我连读和写都不会,我怎么能找到工作呢?
4、表示对比，主要有以下情况：
A、 意为“既然”，相当于since, 如:
How can they expect to learn anything when they never listen?
既然他们从不听讲，他们怎么能指望学到东西呢？
How can I help them to understand when they won't listen to me?
既然他们不愿听我的，我又怎么能帮助他们弄明白呢?
B、意为“然而”，相当于whereas， while，如：
They have only three copies when we need five.
他们只有三本，而我们却需要五本。
She claimed to be 18, when I know she's only 16.
她自称是18岁，可是我知道她才16。
C、when 从句中使用虚拟语气，意为“本来……却……”。如:
She paid when she could have entered free.
本来可以免费入场，而她却付了款。
She stopped trying when she might have succeeded next time.
她本来下次有可能获得成功，但她却不再尝试了。
They were gossiping, when they should have been working.
他们本该工作，却在那里说别人的闲话。
5、由 when 等引导的状语从句，如果主语与主句的主语一致或是 it，谓语动词又含 be 动词时，主语和 be 动词常常被省略，就变成“ when + V-ing/ V-ed/ adj/ 介词词组”的结构。 如:
when + V-ing:
When leaving the garden, a worker was encouraged to return the hoe.
离开花园时，一个工人被鼓励把锄头还给他。
Don't reach sideways when standing on a ladder.
站在梯子上时，不要侧着身子去够东西。
When eating a whole cooked fish, you should never turn it over to get at the flesh on the other side.
吃一整条鱼时，切忌把它翻过来去吃另一面的肉。
when + V-ed:
Emerald starlings are fairly small when compared with other such birds.
与同类的其他鸟相比，翠辉椋鸟要小很多。
When (the museum is) completed , the museum will be open to the public next year.
当博物馆完成，该博物馆将于明年向公众开放。
when + 介词词组 ：
When in Rome, do as Romans do.
入乡随俗。
When in doubt, call the doctor.
如有疑问，请联系医生。
when + adj. :
When (we are ) young, we are full of hope.
年轻时，我们大家总是充满希望。
When upset, I go shopping.
当我不开心的时候，我就购物。