名词性从句在句子中起名词作用的从句叫名词性从句。
名词性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，因此根据它在句中不同的语法功能，名词性从句又可分为主语从句、宾语从句、表语从句、同位语从句。
引导名词性从句的词叫做连接词，包括：1. 连词（在名词性从句中只起连接作用，不作成分）2. 连接代词（在名词性从句中作主语、宾语或表语）3. 连接副词（在名词性从句中作状语）所有的名词性从句的语序都是陈述句语序。
主语从句：是在主句中作主语的从句。
它由连接词引导，通常位于谓语动词之前。
连接词：1. 连词：在从句中不充当任何成分，常见的引导主语从句的连词有that, whether。
That he got the first prize excited him much.他获得一等奖一事使他非常兴奋。
It is doubtful whether we can get there on time.我们能否按时到达那儿还不确定。
1）that引导的主语从句：在从句中不作任何成分，也没有实际意义。
一般情况下that不可省略。
That light travels in straight lines is known to all.众所周知，光沿直线传播。
That you don't like him is none of my business.你不喜欢他不关我的事。
2）whether引导的主语从句：常置于句首，表示“是否”之意；if一般不引导主语从句。
Whether he can finish his task on time is of greatimportance.他是否能按时完成任务非常重要。
Whether you can succeed or not depends on how hard you work at it.你能否成功取决于你努力的程度。
连接代词：在从句中起名词或代词的作用，常作从句的主语、宾语、表语或定语，有具体意义且不能省略。
常见的连接代词有who, whom, whose, what, which,whoever, whomever, whatever, whichever。
What we should do with the problem is undecided.我们如何处理这个问题还未作出决定。
（what引导主语从句，不表疑问意义）Whom the letter was from is still unknown.这封信是谁邮寄的还不清楚。
连接副词：在从句中起副词的作用，常作从句的状语，表示时间、地点、原因、方式等。
常见的连接副词有when, where, how, why, whenever, wherever, however等。
Why he did that wasn't quite clear.他为什么做那件事还不十分清楚。
Where we can look up his address is still a problem.我们在哪儿可以查他的地址还是个难题。
注意：it代替主语从句作形式主语that从句作主语通常用it作形式主语，而将that从句置于句末。
常见的it替代主语从句的句式主要有以下几种：1）It + 系动词 + 形容词 + that从句It is quite clear that the whole project is doomed to failure.很清楚，整个计划注定要失败。
It is likely that there will be a snowstorm tomorrow.明天很可能会有一场暴风雪。
2）It + 系动词 + 名词 + that从句常见的用于此结构的名词有：a pity（遗憾），a shame（遗憾，耻辱），a fact（事实），an honor（荣耀），a wonder（奇迹），no wonder（难怪），good news（好消息）等。
It's no wonder that you've achieved so much success.难怪你已取得了如此大的成功。
It's a pity that you didn't attend their wedding.你没有参加他们的婚礼真是遗憾。
3）It + 系动词 + 动词的过去分词 + that从句It is said that no passengers were injured in the accident.据说在那次意外事故中没有乘客受伤。
It is announced that the plan has been successfully carried out.据宣布计划已被顺利实施。
4）It + 特殊动词+ that从句常用于这种结构的动词有：seem（看上去），appear（显得），happen（碰巧），matter（关系重大），turn out（结果），occur to sb. （某人突然想起）等。
It seems to me that he objects to the plan.在我看来，他好像反对这个计划。
It happened that I met an old friend on the street yesterday.昨天我碰巧在街上遇见了一位老朋友。
II 宾语从句：在主句中起宾语作用的从句称为宾语从句。
宾语从句可分为动词的宾语从句、介词的宾语从句和形容词的宾语从句。
1. 连词that, if/ whether引导的宾语从句引导宾语从句时，that, whether/if 都只起连接作用，在从句中不作任何成分。
that无实义，在句子中可以省略；Whether/if 意为“是否”，不可省略。
He said (that) he couldn't finish his work before dusk.他说黄昏之前他完不成任务。
Nobody knows whether/ if he likes school or not.没有人知道他是否喜欢上学。
注意：whether/ if都意为“是否”。
一般情况下，它们可以互换，口语中常用if，但在以下情况中，只能用whether：1）与or not 紧接连用时。
如：Let me know whether or not you can come.请让我知道你是否能来。
2）作介词的宾语从句时。
如：We are interested in whether you will attend the meeting.我们对你是否会参加会议感兴趣。
3）引导主语从句、表语从句与同位语从句时4）在动词不定式前时。
如：I don’t know whether to go.2. 连接代词引导的宾语从句who, whom, what, which, whomever, whoever, whatever, whichever 等引导宾语从句，并且在宾语从句中充当主语、宾语、表语、定语等成分, 每个连接代词都有自己特定的意义，一般不省略。
Do you know who is in charge of the company你知道谁负责这家公司吗（作主语）Can you tell me which boy is your son你能告诉我哪个是你儿子吗（作定语）What do you think his job is你认为他是干什么工作的（作表语）连接副词引导的宾语从句when, where, why, how, whenever, wherever, however等引导宾语从句，并且在宾语从句中充当状语，表示时间、地点、原因、方式等。
（有时how much, how many等也可引导宾语从句。
每个连接副词都有自己特定的意义，一般不省略。
）The police asked me how the accident happened.警察问我事故是怎么发生的。
（方式状语）I don't know when we will meet again.我不知道我们何时才能再见面。
（时间状语）Do you know where Mr. Brown lives你知道布朗先生住在哪儿吗（地点状语）使用宾语从句的注意事项宾语从句的语序：在宾语从句中要用陈述句语序。
He asked me when we could set out the next day.他问我第二天什么时候可以出发。
Did you find out where she lost her car你查出她在哪里丢的车了吗宾语从句的时态：1）如果主句的谓语是一般现在时，其宾语从句的时态可以是任何适当的时态。
She says (that) she works from Monday to Friday.她说她周一至周五上班。
（从句时一般现在时）She says (that) she will leave a message on his desk.她说要在桌子上留个便条。
（从句是一般将来时）She says (that) she has never been to Mount Emei.她说她从来没有去过峨眉山。
（从句是现在完成时）2）如果主句的谓语是一般过去时，其宾语从句的时态一般要用适当的过去时态（客观真理、事实除外）He said there were no classes yesterday afternoon.他说昨天下午没有课。
（从句是一般过去时）He said that he was going to take care of the baby.他说他会去照看这个婴儿。
He said that they were having a meeting at that time.他说他们那时正在开会。
（从句是过去进行时）3）当宾语从句是表达客观真理和规律的句子时，其时态仍用一般现在时。
The teacher told us that nothing is difficult if we put our heart into it.老师告诉我们，世上无难事，只怕有心人。
He said that light travels much faster than sound.他说光比声传播得快。
解题技巧：1. 动词find, feel, think, consider, make, believe, guess, suppose, assume等后有宾语补足语时，则要用it作形式宾语，而将that 宾语从句后置。
I think it necessary that we take plenty of boiled water every day.我认为每天多喝开水是有必要的。
