在英语中，句子可分为简单句和复合句，复合句即常说的“从句”。由一个主句和一个（或几个）从句构成的句子叫主从复合句。在复合句中，我们是根据从句在整个句子中所充当的成分来给复合句命名的。如：主语从句，宾语从句，定语从句，等等。所有的复合句中，“主语从句，宾语从句，表语从句，同位语从句”，他们的从句有名词性特征，因此合称为“名词性从句”。
名词性从句的引导词是否在名词性从句中充当成分的情况
充当成分： 连接代词— what, which, who, whom, whose，whatever, whoever, whichever,
不从当成分： 连接副词— when, where, why, how, however
只起连接的作用：that, if/whether, because（that引导的名词性从句，既不作任何成分，也无意义，只是表明从句是一种陈述语气，起个连接作用；if不能引导表语从句和同位语从句，if引导主语从句时不能置于句首，if引导宾语从句时，只能置于及物动词后，不能置于介词后；because只能引导表语从句）
㈠ 主语从句（用作主语的从句叫做主语从句）
1．that从句作主语
That引导的主语从句可位于句首直接作主语;也可以位于句末，用it作形式主语;
① That we are invited to a concert this evening is a piece of good news for us.
② It is obvious that the earth is round.
2．whether/if（是否）从句作主语同样可放句首或句末，但放句首只能用whether，不能用if.
① Whether we shall go or stay ,is uncertain.
② It was not known whether/if he would ask.
3． whoever, whichever, whatever, however引导主语从句只能放句首。
① Who will be sent abroad has not been decided.
② It is a mystery how you are going to do it.
③ It is not yet clear who was responsible for the accident.
④ It is his own affair where he goes on Sunday.
⑤ Whoever is here gets a prize.
㈡ 表语从句（用作表语的从句叫表语从句，是对主语进行解释，使主语内容具体化。
① My trouble is that I do not have much experience in this kind of work.
② My suggestion is that we should do this by turns.
1.That引导表语从句，不可省略that; wh-引导的从句作表语。（分别在句中作相应成分，不可用if引导）
① That is what we should do. That is where our basic interest(利益) lies.
② The trouble is that he misunderstood me
③ The problem is who should be responsible for the project.
④ That is how they learned a foreign language.
⑤ The city is no longer what（过去的情形或样子）it was ten years ago.
2.当句子的主语是：advice, demand, decision, desire, idea, insistence, instruction, order, plan, proposal, recommendation, request, requirement, suggestion, wish等名词时，表语从句须用虚拟语气，即：should +do。如：
My advice is that you should go to apologize to him. 我的建议是你应该去向他道歉。
㈢ 宾语从句（用作宾语的从句叫宾语从句），既可作及物动词的宾语，也可作介词的宾语。
1.作及物动词的宾语：
①They demanded that he should leave the country within 24 hours.
②Could you tell me where the post office is?
③We must find out who did all of this.
④I do not know whether/if those figures are correct.
2.当宾语太长时，为了使句子保持平衡，通常用it作形式宾语，真正的宾语置后。
① I think it puzzling when the bank was robbed.
② I put it to you whether/if that is the best solution of the problem.
有些动词带宾语从句时需要在宾语与从句前加it（双宾语）
这类动词主要有：hate, take, owe, have, see to. award, lend. hand, mail, offer, pass, pay, post, read, return, show, teach, tell
如：I hate it when they say with their mouths full of food. 我讨厌他们满嘴食物时说话.
3.若宾语从句是wh-类，则不可用it代替
We all consider what you said to be unbelievable.
我们都认为你所说的是不可信的。
4.作介词的宾语：（介词后的宾语从句不可用if引导）
This thing depends on whether he is coming or not.
5.介词之后一般跟that引导的宾语从句，故需用it作形式宾语。
① You can write about it that people should protect our planet.
② I am sorry for what I have said.
③ He said nothing about how the work was to be done.
6.某些形容词（常用certain/sure/afraid）后也可以接宾语从句（由that引导的较多）
① I am sure that he will pass the exam.
② He was surprised that his friends would not believe him.
7.在宾语从句中用于 suggest, propose, move, insist, desire, demand, request, order, command 等动词后的宾语从句中，表间接的命令和建议。其谓语形式是"(should) + 动词原形"。如：
I suggest that we (should) go swimming.
㈣ 同位语从句（与其前的某名词可以相等，说明该名词所表示的内容）
1.这些词通常是：
fact,news,idea,thought,problem,question,reply,report,remark,order,instruction,impression,suggestion，truth, belief, condition, doubt, hope…
2.同位语从句可用that引导，wh-引导，不可以用if和which引导。
① The news that he intended to come gave us much pleasure.
② The question whether we need help has not been considered.
③ The fact has to be faced that the nearest gas station is 30km away.
3.当句子的主语是：advice, demand, decision, desire, idea, insistence, instruction, order, plan, proposal, recommendation, request, requirement, suggestion, wish等名词时，同位语从句须用虚拟语气，即：should +do。如：
His advice is right that we should help the disabled.
注：以下情况的主语从句只能用it作形式主语
1.It is said/reported/believed/certain… it happens/occurs… it is does not matter… 含主语从句的复合句是疑问句时：is it important what he said is right?
2.并列的宾语从句，且由that引导时，只可以省略第一个that.
3.在名词性从句中，都可以用that引导，但that都无含义，且不作成分。然而只有在宾语从句和放句中的主语从句中that才可以省略。
4.任何的复合句中，从句的语序须是陈述语序。
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