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 whensomeone 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/Whenshe 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/Thoughhe 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。
3.as引导时间状语从句时,常表示两个动作交替进行或同时发生,可译作"一边......一边......;随着"。
He hurried home, looking behind as he went.
他匆忙回家去,一边走一边回头望。
Astime went on, it's getting warmer and warmer.
随着时间的推移,天越来越暖和了。
4.by the time引导时间状语从句时,一般情况下,如果从句用一般过去时,主句则用过去完成时;如果从句用一般现在时,主句则用将来完成时。
By the time Igot to the station, the train had alreadygone.
我到达火车站时,火车已经开走了。