名词性从句在句子中起名词作用的句子叫名词性从句。
名词从句的功能相当于名词词组, 它在复合句中能担任主语、宾语、表语、同位语等，因此根据它在句中不同的语法功能，名词从句又可分别称为主语从句、宾语从句、表语从句和同位语从句。
概说引导名词性从句的连接词引导名词性从句的连接词可分为三类：连接词：that, whether ,if （不充当从句的任何成分）连接代词：what, whatever, who, whoever, whom, whose, which连接副词：when, where, how, why 以及由how 构成的词组如how many ,how much, how soon, how often ,how far ,how long, how tall ,how high等.1,名词性that-从句1）由从属连词that引导的从句叫做名词性that-从句。
That只起连接主句和从句的作用，在从句中不担任任何成分，本身也没有词义。
名词性that-从句在句中能充当主语、宾语、表语、同位语。
例如：主语：That he is still alive is sheer(全然的) luck.他还活着全靠运气。
动词宾语：John said that he was leaving for London on Wednesday. 约翰说他星期三要到伦敦去。
形容词宾语：I am glad that you are satisfied with your job. 你对工作满意我感到很高兴。
介词宾语: I know nothing about him except that he is from the United States.除了知道他是从美国来之外,我对他全无了解表语：The fact is that he has not been seen recently. 事实是近来谁也没有见过他。
同位语：The fact that he has not been seen recently upsets everyone in his office. 近来谁也没有见过他，这一情况令办公室所有的人不安。
2 if, whether引导的名词性从句1）yes-no型疑问从句,从属连词if, whether引导的名词从句是由一般疑问句或选择疑问转化而来的，因此也分别被称为yes-no型疑问句从句和选择型疑问从句，其功能和that-从句的功能相同。
例如：主语：Whether the plan is doable remains to be proved. 这一计划是否可行还有等证实。
宾语：Let us know whether / if you can finish the article before Friday.请让我们知道你是否能在星期五以前把文章写完。
形容词宾语：She's doubtful whether we shall be able to come. 她怀疑我们是否能够前来。
介词宾语：I worry about whether he can pass through the crisis of his illness.我担心他是否能度过疾病的危险期。
表语：The point is whether we should lend him the money.问题在于我们是否应该借钱给他。
同位语：They are looking into the question whether the man is trustworthy.他们调查他是否值得信赖。
2）选择性疑问从句选择性疑问从句由关联词if/whether…or或whether…or not构成。
例如：Please tell me whether / if they are Swedish or Danish.请告诉我他们是瑞典人还是丹麦人。
I don't care whether you like the plan or not. 我不在乎你是否喜欢该计划。
3, 名词性wh-从句由wh-词引导的名词从句叫做名词性wh-从句。
Wh-词包括who, whom,. whose, whoever, what, whatever, which, whichever等连接代词和where, when, why how, how many ,how much, how soon, how often ,how far ,how long, how tall ,how high等连接副词。
Wh-从句的语法功能除了和that-从句一样外，还可充当宾语补语和间接宾语等。
例如：主语：How the book will be sold depends on its writer.书销售如何取决于作者本人。
直接宾语：In one's own home one can do what one likes.在自己家里可以随心所欲。
间接宾语：The club will give whoever wins a prize.获胜者俱乐部将颁奖。
形容词宾语：I'm not sure why she refused their invitation.我尚不能肯定她为什么拒绝他们的邀请。
介词宾语：That depends on where we shall go. 那取决于我们去哪儿。
表语：My question is who will take over president of the club.我的问题是谁将接任该俱乐部主席职位。
宾语补足语：She will name him whatever she wants to.她高兴给他起什么名字就取什么名字。
同位语：I have no idea when he will return.我不知道他什么时候回来。
主语从句主语从句在句中做主语,例如:That mathematics finds its use in every science is obvious.数学在各个学科中都有用是很显然的That he was chosen made us very excited.他被选上了使得我们很激动Why the earth moves round the sun is quite clear now.地球绕太阳转的原因现在已经很清楚.Whether he was chosen is unknown.他有没有被选上还不知道.What we need is careful preparation.我们需要的是准备.Who will go makes no difference. 谁去都一样.Which team will win the match is still not certain.哪个对要赢还不是很有把握.When they will start has not been decided yet.他们什么时候出发还没有决定.Where she has gone is a mystery. 他到什么地方去了是个秘密How this happened is not clear. 究竟是怎么回事儿还不清楚.Wherever you are is my home ---- my only home.你所在的任何地方就是我的家----我唯一的家。
注意连接代词whoever, whatever, whichever等引导主语从句的含义Whoever comes will be welcome. (whoever=anyone who) 来的人将受到欢迎。
Whoever is tired may rest. (whoever=anyone who)任何累了的人都可以休息Whatever he did was right. (whatever=the thing that) 他所做的事情是正确的。
Whichever of you comes in will receive a prize. (whichever=anyone of you who)在口语中常用whoever代替whomever.Whoever/whomever you invite will be welcome.你们当中不论哪个进来将会得到奖励。
主语从句常放在句尾而用it 做形式主语使句子更加平衡，这时口语中常省略that, 如上面第一句改成“It is obvious (that) mathematics finds its use in every science.”更好。
在下列情况下必须把主语从句放在句尾而用it 作形式主语：1．It+be+过去分词(said, reported, thought, expected, believed, decided, known, announced, arranged, suggested, ordered etc.)+that从句。
如：It is said that Mr. Green has arrived in Beijing. 据说格林先生已经到了北京。
It is reported that China has sent another man-made earth satellite into orbit.据报道中国又成功地发射了一颗人造地球卫星。
It is said that he is a good doctor.据说他是个医生It is suggested that the meeting ( should ) be put off. 建议把会议推迟。
It was ordered that we ( should ) arrive there in two hours。
得到命令我们两小时内赶到那里》2．主句是疑问句或感叹句时，如：Is it true that the film star will come?How wonderful it is that we'll visit the Great Wall tomorrow.Does it matter much that they will not come tomorrow? 他们明天不来很要紧吗？How strange it is that the children are so quiet! 孩子们这么安静真奇怪！3．主句谓语动词是appear,seem, turn out, happen, occur, come, strike, follow, matter 等时，如：It （so）happened that she had a cold and didn't go with us that day.It turned out that I was wrong.It seems that this test is reliable(可靠的).4．It+be+形容词(obvious, true, natural, surprising, good, wonderful, funny, possible, (un)likely , (im)possible, certain, probable, etc.)+that从句。