3.复合句
由一个主句和一个或一个以上的从句构成的句子叫复合句。主句是句子的主体，从句是句子的一个成分，不能独立使用。下面总结一下初中阶段的主要从句。
1)宾语从句
宾语从句在复合句中作主句的宾语。引导宾语从句的连接词有以下几种形式：
(1)由that引导，只起连接作用，不充当句子成分，没有实际意义，可以省略。如：
We know (that) there are two kinds of sports.
He decided (that) he was not going to say anything about it
(2)由whether或if(是否)引导，只起连接作用，不充当句子成分，但不能省略。如：
No one knows whether/if they have decided on the date of the meeting.
(3)由连接代词who，whom，what，which，whose和连接副词when，where，why,how等引导的宾语从句，这些连接词除了起连接主句和从句的作用外，还可在句中充当某个成分，不能省略。如：
Could you tell me how I can get to the nearest subway station?
She told me what she had seen in the museum.
(4) 由it作形式宾语的从句。如：
I think it necessary that everybody should try his best to help fight pollution.
(5) 由形容词sorry, afraid, sure, glad等引出的宾语从句。如：
We were surprised that Peter was the first to come.
I am sure that you will be a success in the future.
2)状语从句
在复合句中作状语的句子叫状语从句。常用状语从句有以下几种类型：
(1)由when, while, as, since, until, as soon as等连词引导的时间状语从句。如：
It's more than ten years since they came to Shanghai.
Please send me an E-mail as soon as you arrive in Switzerland.
(2)由if，unless等词引导的条件状语从句。如：
We shall go for a picnic if it doesn't rain tomorrow.
I won't buy the dress unless it fits me well.
(3) 由because, since, as, now that等词引导的原因状语从句。如：
He asked for leave because he had to see the dentist.
Now that everyone is here, shall we begin our discussion?
(4)由though/although引导的让步状语从句。如：
Although it was very late, he went on working.
(5)由so that, in order that引导的目的状语从句。如：
He ran as fast as he could so that the people in Athens could learn the news earlier.
(6)由so... that, such... that等词引导的结果状语从句。如：
He ran so quickly that we couldn't keep up with him.
The bus broke down, so we had to walk to the cinema.
English is such a useful language that it is spoken in many countries.
(7) 比较状语从句。常用的句型有as... as与more than结构。(参见形容词、副词一章)
3)定语从句
在复合句中作定语的从句称为定语从句。定语从句一般直接跟在被修饰的名词之后。被修饰的名词称为“先行词”。定语从句一般由关系代词who, whom, that, which引导，它位于先行词与定语从句之间，本身又在定语从句中充当某个成分。如：
The boy who/that is reading under the tree is my elder brother.
先行词 关系代词作主语
The present which/that you gave me for my birthday is very nice.
先行词 关系代词作宾语
(1) who和whom指人，who在从句中作主语，不能省略。而whom在从句中作宾语，一般可以省略。如：
The woman who will give us a talk tomorrow is a famous professor.
Do you know the girl (whom) we met outside the school gate?
(2) which指物，在定语从句中可以作主语或宾语。作宾语时可以省略。如：
The story which tells about three monkeys is very interesting.
The magazine (which) I borrowed from the library was newly published.
(3) that指人或物，在定语从句中作主语或宾语。作宾语时可以省略。如：
The pictures that hang on the back wall were all painted by her.
The restaurants (that) they have set up are welcomed by children.
(4)如果先行词被序数词、形容词最高级、不定代词all, everything, nothing等以及only修饰时，关系代词必须用that。如：
All that we need is fresh water.
I'll never forget the first job that I found.
Judy is the most honest girl that I have ever seen.
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