1、名词性从句在复合句中起名词作用的句子叫名词性从句。
名词性的功能相当于名词词组，它在句子中担任主语，宾语，表语，同位语，介词宾语等，因此根据句子中的不同语法功能，可分为主语从句，宾语从句，表语从句，同位语从句。
2、连接词名词性从句主要有四种结构：以that引导的从句；以whether/if引导的从句；以特殊疑问此引导的从句；以what或wh-ever等连接的从句。
此外，as if，as though，because，as也可以引导表语从句。
（1）从属连接词引导名词性从句的从属连接词主要有that，whether，if。
○1That只起连接主句和从句的作用，在从句中不充当任何成分，也无词义。
○2whether/if起连接主句和从句的作用，在从句中不充当任何成分，本身有词义，表示“是否”。
（1）特殊疑问词○1连接代词：who，whom，whose，what，which。
起连接主句和从句的作用，在从句中充当成分，本身有词义。
○2连接副词：when，where，how，why。
起连接主句和从句的作用，在从句中充当成分，本身有词义。
（3）名词性关系从句：○1连接代词：whoever，whomever，whatever，whichever。
○2连接副词：whenever，wherever，however。
（4）从属连词as if，as though，because，as引导的表语从句。
连接词的用法1、thatThat只起连接主句和从句的作用，在从句中不充当任何成分，也无词义。
（1）主语从句○1可用it作形式主语，从句本身置于句末，常见的句型有：A、It+形容词+ that从句It is clear that he was telling the truth.B、It+名词词组+that从句It is a pity that you can’t go with us.C、It be+过去分词+that从句It’s said/reported/believed/thought/expected thatIt’s said that he has been there many times.D、It+及物动词+宾语+that从句It worried her a bit that her hair was turning grey.It shocked me that he didn’t tell anybody where he was.E、It seems/happens/appear等不及物动词+that 从句It seems that he was lost something.○2在口语和非正式文体中，that常可以省略，尤其实在非常短的句子中，that是不必要的；但that从句位于句首时，连词that是不可以省略的。
It’s a pity（that）you are leaving.That we are invited to the concert this evening is good news to us.2、宾语从句○1常见的可以接从句作宾语的动see，say，know，discover，believe，tell，show，think，consider，sure等，在可以接复合宾语（宾语+宾补）的动词后，如think, make, consider等，可以用it 作形式宾语，而把真正的宾语从句后置，that 在这种情况下不能省略。
We heart that one more person died of ill in Mexico.I made it clear that I was determined to carry out the plan.○2that从句一般不能充当介词宾语，偶尔可以作expect，in的宾语。
He is a good student expect that he is a little bit careless.He differed from his classmates in that he devoted his spare time to reading.○3其它介词后面需要that从句作宾语时，必须用it作形式宾语。
You may depend on it that I shall always you.你要相信我会一直帮着你。
3、表语从句that不可以省略。
这种从句往往对主句的内容起进一步解释作用。
其基本结构为“主语+系动词+ that从句”。
My decision is that all of us are to start at 6 o’clock tomorrow.4、同位语从句that不可省略。
如果同位语从句意义完整，应用that引导同位语从句。
5、whether/if（是否）起连接主句和从句的作用，在从句中不充当任何成分，本身有词义，表示“是否”。
（1）主语从句若句中需要表达“是否”意思时，则用whether/if。
当主语从句放在句首时，只能用whether 引导，不用if 引导；当it作形式主语，主语从句放在句末时，whether和if均可以用。
Whether the meeting will be given is still a problem.It’s doubtful whether/if he will come here.（2）宾语从句whether/if引导的宾语从句：作“是否”讲，常用在ask，care，find out，know，wonder等动词之后引出带有疑问意义的宾语从句。
从句仍保持陈述语序，whether或if 不当成分。
I wonder if/whether he can come tomorrow.。
○1在及物动词后（错）I don’t care whether he doesn’t come.（whether从句不用于否定句中）（对）I don’t care whether/if he comes or not.（对）I don’t care whether or not he comes.（错）I don’t care if or not he comes.（对）I don’t know whether to go there.○2在介词后在介词后只用whether，不用if。
It depends on whether you can work do this work well.（3）表语从句只能用whether，不能用if引导。
The problem is whether the meeting will be given.（4）同位语从句只能用whether，不能用if引导。
I have no idea whether the meeting will be given.6、特殊疑问词（1）主语从句○1连接代词who，whom，where，what，which等引导从句时，在主句中作主语，宾语，表语等。
Which one is better will be decided by yourself.○2连接副词when，where，why，how等引导从句时，在主句中作状语。
Where they will go is unknown.○3特殊疑问词引导主语从句时，常用it作形式主语。
It makes no difference where we shall have the meeting.It has not been decided yet who will preside over the meet.由谁来主持会议还没有确定。
（2）宾语从句○1连接代词who，whom，where，what，which等引导从句时，在主句中作主语，宾语，表语等。
○2连接副词when，where，why，how等引导从句时，在主句中作状语。
○3能接特殊疑问词引导的宾语从句的动词（词组）有很多，常见的有see，tell，ask，answer，know，discuss，imagine等。
I can’t imagine how he did it.They couldn’t understand why I refuse it.○4作介词的宾语It all depends on how we solve the problem.We are worrying about what we should do next.（3）表语从句○1连接代词who，whom，where，what，which等引导从句时，在主句中作主语，宾语，表语等。
○2连接副词when，where，why，how等引导从句时，在主句中作状语.The problem is where we should stay.（4）同位语从句○1连接代词who，whom，where，what，which等引导从句时，在主句中作主语，宾语，表语等。
○2连接副词when，where，why，how等引导从句时，在主句中作状语。
My question how I shall get in touch with him has not been answered.7、名词性关系从句名词性关系从句实际上是先行词与在其后的定语从句的结合。
What是最常用来引导名词性关系从句的引导词，此时what=the thing（s）which/that，有时what可以用作前置定语，如what help，what funny stories等。
此外，whoever=anyone who，whichever=anything that/anyonethat（whichever也可以指人），whatever=anything that（whichever和whatever也可以作定语）。
有时，where（=the place where）和when（=the time when）也可以用来引导名词性关系从句。
（1）主语从句What they need is a good textbook.Whichever he likes will be given to him.Whichever book he bought would be paid for.Whoever did this job must be rewarded.（2）宾语从句She will give whoever（=anyone who）needs help a warm support.She walked up to where（=the place where）he stood.（介词宾语）I can judge by what（=the thing that）I know of him.（介词宾语）You can write about whatever topic（=any topic that）you prefer.（介词宾语）（3）表语从句This is where our problem lies.Tomorrow is when it would be the most convenient.（4）同位语从句I gave the girl a big doll，exactly what she longed to have.（5）名词性关系从句还可以作宾语补足语We’ll make him whatever he is fit for.I’ll call the baby whatever name you like.He has made the company what it is today.whether，不用if。