英语语法（复合句）讲解：状语从句
一、状语从句的定义及分类
状语从句在复合句中修饰主句或者主句中的动词。状语从句由从属连词引导，与主句连接，位于句首时，常用逗号与主句分开，位于句末时，前面一般不用逗号。状语从句根据其用途可以分为：时间状语从句、原因状语从句、结果状语从句、条件状语从句、让步状语从句、地点状语从句、目的状语从句等。
二、时间状语从句
1. when的用法
① when意为“当……时候”，引导时间状语从句，表示主句的动作和从句的动作同时或者先后发生。
I feel very happy when you come to see me.
When you are crossing the street, you must be careful.
② when引导的时间状语从句中的动词可以用延续性动词或者非延续性动词。
I worked for a foreign company when I was in Shanghai.
2. while的用法
while引导的时间状语从句，常常翻译为“与……同时、在……期间”，while引导的从句常用延续性动词或表示状态的动词。
They rushed in while we were discussing problems.
My mother was cooking while I was doing my homework.
3. as的用法
as引导时间状语从句时可以表示“正当、一边……一边……、随着”等意思。
Helen heard the story as she washed clothes.
We get wiser as we get older.
4. before/after的用法
before表示“在……之前”，after表示“在……之后”。
I will always work hard before I enter Peking University.
She showed me many beautiful stamps after I got to her home.
5. until/till的用法
until和till都表示“直到”，常可互换，但till不用于句首。
① 主句谓语动词是延续性动词时，主句用肯定形式，表示这一动作或者状态一直持续到until/till引导的从句的动作发生为止。
I'll wait for you until you come to see me.
② 主句的谓语动词是非延续性动词时，主句用否定形式，表示主句的动作直到until引导的从句的动作发生才发生，构成not...until结构，表示“直到……才……”。
I didn't go to bed until I finished my homework.
6. since的用法
since引导的时间状语从句，表示“自……以来”，主句用现在完成时，从句用一般过去时。
I have worked in this company since I graduated from Peking University.
7. as soon as的用法
as soon as引导的时间状语从句，表示从句的动作一发生，主句的动作随即发生，意为“一……就……”。该从句经常用一般现在时表示将来。
As soon as he arrives, I'll tell him.
三、条件状语从句
主句为祈使句、一般将来时或含有情态动词时，条件状语从句用一般现在时。
1. if的用法
if引导条件状语从句，表示“如果、假如”。
I'll visit the Great Wall if it doesn't rain tomorrow.
2. unless的用法
unless引导条件状语，表示“除非、如果不”，相当于if not。
We'll go for an outing tomorrow unless it rains.
四、原因状语从句
原因状语从句通常用because, since或as引导，这三个词所表示的语气由because到as逐渐减弱。
1. because的用法
because表示因果关系时，语气最强，用来回答why的提问，所表示的原因往往是听话人所不知道或者感兴趣的。
I'm leaving because I am fed up with the boss.
My friends admire me because I am handsome and successful.
2. since的用法
表示人们已知的事实，不需要强调的原因，因此常常翻译为“既然……”，通常放在句首。
Since Monday is Bob's birthday, let's give him a party.
Since everyone is here, let's get started.
3. as的用法
as与since差不多，所引出的理由在说话人看来已经很明显，或已为说话人所熟悉。
She didn’t hear us come in as she was asleep.
五、目的状语从句
目的状语从句通常由so that，in order that等引导，谓语中常含有may, might, can, could, will, would等情态动词。
He must get up early so that he can go to work on time.
六、结果状语从句
1. 由so...that, such...that引导
so...that与such...that引导结果状语从句时，在结构上有所不同：
so+形容词/副词+that
such+a/an+形容词+单数名词+that
such+形容词+复数名词+that
such+形容词+不可数名词+that
so+形容词+a/an+单数名词+that
注意：当名词前面有many, much, little, few修饰时，用so而不用such。
She is such a lovely girl that we love her very much.
=She is so lovely a girl that we lover her very much.
We have so much time that we can finish the work very well.
2. 结果状语从句与不定式结构的相互转换
so...that句型的否定形式可与too...to...或者not...enough to相互转换。
He is so young that he can't go to school.
=He is too young to go to school.
=He is not old enough to go to school.
七、让步状语从句
1. though/although的用法
although和though意义一样，意为“虽然、尽管”，都表示让步，一般情况下可以互换，两者可以与yet连用，但是不与but连用。
They are generous though they are poor.
Although he was a Japanese, he spent most of his life in China.
2. even though/even if的用法
even though和even if都是“尽管、即使”的意思，表示语气更强的让步。
Even though he's 24 now, he's still like a little child.
We'll make a trip even if/even though the weather is bad.
八、地点状语从句
引导地点状语从句的从属连词主要是where和wherever。
He lives where the climate is cool.
Where there is a will, there is a way.