时间状语从句用来说明主句谓语动词发生的时间，由连词引导。本单元主要学习when, while, as及by the time引导的时间状语从句。
1．when可以和延续性动词连用，也可以和短暂性动词连用，主句和从句中的动作或状态可以同时发生，也可以先后发生。
When they heard the news, they were very excited.
当他们听到这个消息时，非常激动。(hear是短暂性动词)
When I lived there, I used to go to the seashore on Sundays.
我住在那里时，星期天常到海滨去。(live是延续性动词)
[名师点津]
(1)when还可作并列连词，意为“就在这个时候”。
I was watching TV when someone knocked at the door.
我正在看电视，这时突然有人敲门。
(2)when还可译为“这时；就在那时”，常用于以下句型：
①be about to do... when 正要做……这时
②be doing... when 正在做……这时
③had done... when 刚做完……就
2．while引导时间状语从句时，从句的谓语动词必须是延续性动词。
While/When she was reading a novel, she cried.
她看小说的时候哭了。
They rushed in while we were discussing problems.
我们正在讨论问题时，他们冲了进来。
[名师点津] while还可以引导让步状语从句，意为“尽管”。另外， while还可用作并列连词，表示“然而”。
While/Although/Though he is young, he has been to many countries.
虽然他很小，但他去过很多国家。
Some men were rich, while others are poor.
一些人很富有，而其他人很贫穷。
