when作为从属连词,在句子中引导状语从句,它有多种不同的表达:

1. when表示时间,意为“当……时;在……的时候”。如:
1)When you see him, please say hello to him.见到他时,代我问他好。
2)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。

2. when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:
1)When I was in Japan, I bought some beautiful pearls. 我在日本时,买了一些漂亮的珍珠。
2)When I reached the station, the train had left. 当我到达火车站时,火车已经开走了。

3.when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”;如:
1) 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。
2) 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。
3) We were about to start when it began to rain. 我们刚要出发就开始下雨了。 
4) We had hardly fallen asleep when the bell rang. 我们刚刚入睡,铃声就响了起来。
5) He had scarcely arrived when he had to leave again. 他刚刚到达就又要离开了。

4.when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?

5. when表示对比,相当于 whereas, while, since, 意为“既然;然而”。
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?

6.when 从句中使用虚拟语气,意为“本来……却……”。例如:
She paid when she could have entered free. 本来可以免费入场,而她却付了款。
  
7.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 in Rome, do as Romans do. 入乡随俗。
When (we are ) young, we are full of hope. 年轻时,我们大家总是充满希望。

8. When在句子中的位置灵活多样,它作为关系副词时,在其引导的定语从句中充当状语,例如:
I still remember the day when I first met Jennifer.我仍然记得第一次见到詹妮弗的那一天。