状语从句是一种副词性从句，通常由一个从属连词或一个起连词作用的词组引导。状语从句可以表示时间、地点、原因、目的、结果、条件、方式、让步和比较9种意义。
（一）时间状语从句
1.when, whenever, while, as所引导的时间状语从句
①When I came into the room, he was writing a letter.当我进屋时，他正在写信。
②We shall go there whenever we are free.我们什么时间有空，我们就去那里。
③I was walking along the street when suddenly someone patted me on the shoulder from behind.
我正在街上走着，这时忽然有人从后面拍我的肩膀。
④While it was raining, they went out.下雨的时候，他们出去了。
⑤I stayed while he was away.他不在的时候我在。
⑥He hurried home, looking behind as he went.他赶快回家，不时地一边走一边向后看。
2.before,after, since所引导的时间状语从句
①Be a pupil before you become a teacher.先做学生，再做先生。
②He arrived after the game started.比赛开始后，他才到。
③Great changes have taken place in China since 1978.自从1978年以来中国发生了巨大的变化。
3.till和until所引导的时间状语从句
