状语从句
在复合句中，用作状语的从句叫状语从句。状语从句根据它表达的意义可以分为：时间状语从句、地点状语从句、原因状语从句、结果状语从句、让步状语从句、条件状语从句、目的状语从句、比较状语从句、方式状语从句。
（1）时间状语从句
引导时间状语从句的连词或短语有：when, while, as, before, after, since, until, as soon as, once, each time, next time等。
1.When引导的时间状语从句
When引导时间状语从句时，意为“当....时”，表示主句的动作与从句的动作同时或先后发生。
例：When you are doing your homework, you must be careful.
Someone knocked（敲）at the door when I was sleeping.
When she first saw the robot, she felt astonished（惊讶）.
When Tom got home, he wept with anger.
注意：when引导的时间状语从句中的动词可以是延续性的，也可以是非延续性的。
2.while引导的时间状语从句
while引导的时间状语从句，意为“与...同时；在...期间”
例：They entered the room while we were discussing problems.
Father was cleaning the car while I was playing computer games.
When she was in college, she felt in love with Mike.
注意：while引导的时间状语从句中的动词必须为延续性或者表示状态的动词。
3.as引导的时间状语从句
as引导的时间状语从句，表达“正当...，一边...一边...”强调主从句动作同时发生。
例：I heard their voice as I cross the hall（走廊，过道）.
As she sang, the tears ran down her cheek（脸颊）.
As I waited at the stop, I heard a big noise.
4.before引导的时间状语从句
before意为“在...之前”,引导时间状语从句，表示主语的动作在从句前发生。例：We cleaned the classroom before we left school yesterday.
It’ll not be long before you regret（后悔）what you’ve done.
5.after引导的时间状语从句
after意为“在...之后”,引导时间状语从句，表示主语的动作在从句后发生。例：He called me after he had finished his homework.
She usually takes a walk after she has dinner.
6.since引导的时间状语从句
Since常意为“自从......”主句常用现在完成时，从句用一般过去时。
例：Everything has changed since I was here.
I haven’t heard from him since he moved away.
常用句型：It is/has been+时间段+since从句，意为“自从....已经多少时间”
例：It has been three years since I moved to Beijing.
自从我搬到北京已经有两年的时间了。
7.till和until导的时间状语从句
Till和until用于肯定句时，主句动词为延续性动词，意为“直达...才”表示“某动作或状态一直持续到某时间点才停止”；用于否定句时，主句动词是非延续性动词。
注意：无论用于肯定或否定，从句都为肯定句。
例：I will stay here until you come back.
He won’t go to bed until she returns.
8.as soon as引导的时间状语从句
as soon as 意为“一...就...”表示从句的动作一发生，主句的动作马上发生。例：As soon as he heard the news（消息，新闻）, he jumped with joy.他一听说那个消息，就开心地跳了起来。
I’ll write to you as soon as I get there. 我一到那儿就给你写信。
He contacted（联系）us as soon as he arrived in Shanghai. 他一到上海就和我们取得联系。
9.by the time引导的时间状语从句
by the time意为“到...时候”，由by the time引导的时间状语从句要注意时态的变化：一般情况下，如果从句的谓语用一般过去时（did），那么主句的谓语用过去完成时（had done）。
例：By the time you came back, I had finished this program.
By the time I arrived at the bus station, the bus had already left.
（2）原因状语从句
引导原因状语从句的连词主要有：because, as, since, for, now that等。
1.because引导的原因状语从句
because引导原因状语从句意为“因为”，表因果关系的语气最强，用来回答why的问句，because引导的原因状语从句往往比主句显得更重要。
例：I stayed at home, because it rained.
Tom didn’t come to school, because he is ill.
We won the game, because we have strong players.
2.since引导原因状语从句
Since引导原因状语从句表示人们已知的事实，不需强调原因，常译为“既然”。Since引导的从句是次要的，主要强调主句的内容。
例：Since he was busy, he didn’t come.
Since you are free today, why not play chess with me.
3.as引导的原因状语从句
as和since的用法差不多，所引出的原因对于说话者看来已经很显然，或已成为听话人所熟悉的，不需要用because加以强调。
例：As Monday is the National Holiday（国庆节）, all government offices will be closed.
I have to speak English with them as they don’t know any Chinese.
4.for引导的原因状语从句
for引导的原因状语从句只能跟在主句的后面，相当于汉语的“其理由是”。例：I stopped to rest, for I was very tired.
