状语从句是在复合句中修饰主句中的动词、形容词、副词或者整个主句的句子。状语从句由从属连词引导，从属连词在状语从句中不充当句子成分。状语从句可分为时间、原因、条件、目的、结果、比较、方式、地点状语从句和让步状语从句。
（1）由从属连词when引导的时间状语从句。When意为“当……的时侯”，when既可以表示一段时间，一个持续性的动作，也可以表示时间的一个点，一个短暂性的动作。主、从句的动作可以同时发生，从句的动作也可发生在主句之前或之后。例如：
Tom went to work in the factory when he was 14 years old．
Mr Brown had already left when I got to his office．
（2）由until引导的时间状语从句。until／till意为“直到……”，主句的谓语动词必须是持续性动词，而且主从句的谓语动词都用肯定式。“not．．．until．．．”表示“直到……才……”时，主句的谓语动词必须是非持续性动词，而且主句用否定式，从句用肯定式。例如：
I didn't go to bed until my granny came back home．
Let's wait until tomorrow．
（3）由as soon as和since引导的时间状语从句。as soon as意为“一……就……”，一般主句的谓语动词用将来时时，从句的谓语动词用现在时。since意为“自从……以来”，通常从句的谓语动词用一般过去时，而主句的谓语动词用现在完成时。例如：
I'll let you know as soon as he comes back．
Mr Read has taught in that school since he left Canada in 1988．
（4）由从属连词if引导的条件状语从句，主句用一般将来时，从句要用一般现在时表示将来时间。例如：
We'll have a sports meeting if it doesn't rain tomorrow．
（5）由because，as，since引导的原因状语从句。because的语气最强烈，通常用来回答why提出的问句。because引导的原因状语从句经常放在主句之后；since的语气比because弱，陈述的理由往往是对方所知道的，译作“既然”；as的语气最弱，只是附带较明显的原因或理由，as引导的从句常放在主句之前，说明原因或理由，后面的主句说明结果，主从句所表达的内容同等重要。例如：
As it was dark，we stopped working．
Since no one is against it，we'll do that．
Betty didn't go to see the film yesterday because she was ill．
（6）比较状语从句由than，as．．．as，not as（so）．．．as来引导，从句中往往省去与主句相同的成分。例如：My brother is younger than I．He worked as fast as an old worker．
（7）表示结果：so that，so．．．that，such．．．that．其中so是副词，后接副词或形容词，such是形容词，后接名词或名词短语。例如：
The old man was so weak that he couldn't get on the train．
It's such an important match that we can't miss it．
（8）让步状语从句由though，although引导
though表示“尽管”，“虽然”，引导让步状语从句，可以放在主句前或主句后。注意用though就不能用but，用but就不能再用 though。例如：
This TV set is too dear though it gives you a better picture．
这句可以改为：It gives you a better picture，but this TV is too dear．
又如：He is rich，but he isn't happy．
Though he is very young，he knows several languages．
（9）表示目的的状语从句：so that。例如：
He borrowed the money so that he could send his son to school．
