复合句是由两个或两个以上的有主谓结构的句子用从属连词连接起来的句子。其中一个句子是主体,叫做主句,而其他的句子叫做从句。
从句由连接词引导, 它们是:
that, before, whether, if, although, because, as long as, as soon as, since, after, who, which, whom, what, whose, why, where, how,when,
从句尽管有主谓结构,但不能单独成为一个句子,在句中, 从句仅担任某个成分,根据担任的成分从句可分主语从句,宾语从句,表语从句,定语从句,状语从句。
主语从句 在句中起主语作用的句子叫做主语从句.
主语从句一般放在句首。
引导主语从句的词有下列:
that, who, whether, if, what, which, when, where, how, why.
Who cleaned the blackboard is not known yet.
谁擦的黑板还不知道。
What he said is not true.
他说的不是实话.
That he'll come to see us is really great.
他来看我们真是太好啦。
It's very good that he has passed the exam.
他通过了考试太好了。
宾语从句 在句中起宾语作用的句子叫做宾语从句，宾语从句放在动词后面。
引导宾语从句的词有下列。
that, if, whether, what, which, when, where, how, why,
I didn't expect that he had broken the glass.
我没有想到他把玻璃打破了。
Could you tell me who is your teacher?
你能告诉我谁是你的教师吗？
I don't know why he is absent.
我不知道为什么他不在。
表语从句 在句中起表语作用的句子叫做表语从句，表语从句放在系动词的后面。
引导表语从句的词有下列。
that, if, whether, what, which, when, where how, why等.
The question is whether he will join us next time.
问题是下次他是否跟我们一起干。
That is why I am late.
这就是我为什么迟到。
定语从句 在句中起定语作用的句子叫做定语从句，定语从句放在被修饰的名词后面。
引导定语从句的词有下列。
who, whose, that, which, whom, when, where why.
Do you know the man who is in the car.
你认识坐在汽车里的人吗？
This is the place where I was born.
这就是我出生的地方。
That is the reason why he lost his job.
这就是他为什么失去工作的原因。
状语从句 在句中起状语作用的句子叫做状语从句，状语从句一般放在句后,强调时放在句首。
He opened the windows since it was hot.
由于天气热，他打开了窗户。
When he was young,he couldn't go to school。
他年青的时候没能上学。
I'll let you know as soon as he come back.
他一回来我就告诉你。
定语从句相当于形容词，用来修饰名词或代词。
宾语从句相当于一个宾语，用作名词。
状语从句相当于句子中的一个状语，或副词，指时间、地点和原因等。
虽然它们意义不同，但用法大致相同，都要用陈述语序，都要有连接词
名词从句、定语从句
名词从句
名词从句有四类：宾语从句、主语从句、表语从句、同位语从句。例如：
宾语从句：I don’t know where he will go.
主语从句：Where he will go is unknown.
表语从句：The problem is how we can get so much money.
同位语从句：We all feel sorry about the news that he failed in the game.
下面几点是学习名词从句要加以注意的：
1． whether和if都可以引导宾语从句表达“是否”意，（即引导宾语从句时if和whether
可以互换）但以下几种名词从句要用whether（不用if）引导：
（1） 引导主语从句，且在句首时。例：Whether he has won the tennis is not known.(但
It is not known whether/if he has won the tennis).
（2） 引导表语从句时。例The problem is whether he can get a job.
（3） 引导同位语从句时。例：I have no idea whether he will come.
（4） 介词后的whether从句。例：I worry about whether I hurt her feelings.
2． 位于句首引导主语从句的that不可忘。
例如：That he got the first place in the competition surprised all of us.
3． 在“It is suggested/proposed/ordered/a pity/no wonder/necessary/strange/…＋
that从句”结构中，that从句谓语有时用（should）do这样的虚拟语气形式。
例如：It is suggested that we(should) improve the relations between us.
It is a pity that one(should) stay in one place all one´s life.
It is strange that she should have failed to see her own shortcomings（缺点）.
4． 在“The reason why…is that…”句式中that不要误为because.
例如：The reason why he didn´t go to school yesterday is that he was ill.
5． 名词从句中that,what用法比较：
引导名词从句的that是连词，在句中无成份，无意义，而what是连接代词，what＝the
thing(s) that.
例如：It was told in yesterday´s newspaper that what the students had done was praised
by the factory.
I know that he will study.
I know what he will study.
That he works hard at his lessons is known to us all.
What he works hard at is known to us all.
All I ask is that you should tell the truth./He is not what(＝the person that)he
used to be.他和以前大不相同了。
6． where在名词从句中的使用特点：
where在名词从句中有时可以变为“the place where”,有时可以变为“介词＋the place where” 形式。例如：
主从：Where(＝The place where)she has gone is still unknown.
宾从：Would you please tell me where(＝the place where) Mr Smith lives?
表从：Your dictionary remains where(＝in/at the place where)you put.
同位从：Have you any idea where(＝of the place where) she is spending her holidays?
7． 无论是哪种名词从句都是陈述语序。例如：
I know where he lives./Please tell me what her name is.
当一个特殊疑问句本身就是陈述语序（即“主＋谓＋（宾）”或“主＋系＋表”结构），将其变为间接引语（即宾语从句）时，无需改变语序。例如：
What´s the matter with him? She asked me what was the matter with him.
Who looks after your grandfather? He asked me who looked after my grandfather.
参考资料：
http://zhidao.baidu.com/question/6848810.html