在复合句中，修饰主句中的动词、形容词或副词等的从句叫状语从句。状语从句根据它表达的意思可分为时间、地点、原因、目的、结果、让步、比较(或方式)和条件等类。
1.时间状语从句
引导连词有when，while，till，not…until，since，after ,before ,as soon as
The bus won’t start until everybody gets on.
公共汽车直到每人都上车为止，才开动。
When he knocked at the door I was cooking.
当他敲门时，我正在煮饭。
Last night before he came back home, his wife had already cooked dinner and waited for 2 hours.
昨天晚上在他回到家之前，他的妻子已经做好晚饭足足等了他两小时之久。
After I went to church, I went shopping.
2. 地点状语从句
地点状语从句通常由where, wherever 引导。
Where there is a will, there is a way. 哪里有志向，哪里就有出路。有志者事竟成。
Where there is a life, there is a hope. 哪里有生命，哪里就有希望。 留得青山在，不怕没柴烧。
Wherever you go, whatever you do, I will be right here waiting for you.
不管你去哪里，不管你做什么，我都会在此守侯你。
3.原因状语从句
引导连词有because，as, since。
He didn′t see the film because he had seen it.
他没有看那部电影，因为他已经看过了。
They couldn’t get on the train，for it was too crowed.
比较：because, since, as和for
1) because语势最强，用来说明人所不知的原因，回答why提出的问题。当原因是显而易见的或已为人们所知，就用as或 since。
I didn't go, because I was afraid.
Since /As the weather is so bad, we have to delay our journey.
2) 由because引导的从句如果放在句末，且前面有逗 号，则可以用for来代替。但如果不是说明直接原因，而是多种情况加以推断，就只能用for。
He is absent today, because / for he is ill.
He must be ill, for he is absent today.
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