在搞定语法框架和动词时态语态后，我们需要继续攻克的难题就是从句。
正如前文所说，从句包括三大类：名词性从句、形容词性从句和副词性从句。具体从语法学名上看，名从包括主语从句、宾语从句、表语从句和同位语从句。而形容词性从句是定语从句，副词性从句为状语从句。
1. 从句基石
既然同为从句，自然存在共性。把握共性，区别不同，是快速掌握的要诀。简述三点。
第一，本质。从句的本质是为了让表意更为丰富复杂，因此需要用到多个动词，而这多个动词，必须要用衔接词来连接，也就是语法中的“引导词”或“关系词”。
比如That Yao Ming is tall is known to us all. 这句话中存在两个is，所以必须有衔接词，其实就是that。如果我们拆分成单句，就要说Yao Ming is tall. This is known to us all. 这样两个句子，用词更为冗余，结构过于简单，因此才用从句，彰显水平，简洁语言。
第二，引导词。承接第一点，引导词存在的目的就是连接两个或两个以上的动词。为何？因为英语中，两个动词以及两个动词以上要并存一句话，如果没有连词、关系词（引导词）或非谓语，是无法共存的。
也就是说，前面的句子如果变成Yao Ming is tall is known to us all.的话，就是一个错误的句子，两个is没有衔接词语，一山（一句话）出现两虎（两个动词）。
所以，学习从句，最关键的就是把握引导词关系词的使用。这些词语不外乎以下三类：
从属连词：不指代，表承接，比如that，whether/if。
连接代词：指代主语宾语表语，比如who whoever whom whose what whatever which whichever。
连接副词：指代状语，比如when where how why。
了解这些单词词义和用法，基本可以搞定名词性从句了。
第三，主句从句成分完整。我们练习或输出的最终目标是，你讲出来或写出来的句子必须是正确无误的，换言之，必须是主谓宾定状补完整的。这里的规则就是，先确保主谓宾（有时是主系表）完整，再考虑补充定状补。从句中，一般而言关系词或引导词都要充当一定的语法功能或句子成分，你要确定缺的是什么成分或功能，然后对应补充，才能实现从句的正确使用。
举例：I am a boy ____ likes English.
基本思路：找动词——定成分——选答案。
那么可以发现，am和likes是动词，am对应的主语是I，表语（接下系动词后的成分）是a boy，主干成分完整，OK了。likes的主语不知道是谁，宾语是English，所以要补充likes的主语。实际上，主语应该是boy，只不过我们要用who来替换，避免重复。
这就是标准的句子成分分析步骤。而这个例句，就是一个标准的定语从句。翻译为中文是“我是一个喜欢英语的男孩。”
2. 名词性从句的架构
从根本上讲，名词性从句是一整个句子充当一个名词的作用，因此有刚才我们说的四种：主语从句、宾语从句、表语从句和同位语从句。
大家要记住这样一个要点：语法上叫什么从句，则这个从句整体充当那种成分。
比如刚才的例句That Yao Ming is tall is known to us all. 很明显That Yao Ming is tall是从句，因为它有引导词或关系词that，而这样一整个从句相当于后面那个is的主语，所以这个从句就是主语从句。也就是说，“姚明很高这样一个事实，是为我们所知的”。
3. 主语从句
3.1 主语从句，即在复合句中充当主语成分的句子。
That Gu Hongmin mastered 9 languages and gained 13 doctoral degrees surprised us.
3.2 引导词：引导主语从句的连词主要有，不能省略：
从属连词：that whether if
连接代词：who whoever whom whose what whatever which whichever
连接副词：when where how why
3.3 例句（斜体部分为从句）
（1）That he finished writing the composition in such a short time surprised us all.
（2）Whetherwe will go for an outing tomorrow remains unknown.
（3）Who will be our monitor hasn't been decided yet.
3.4 特例
为了防止句子头重脚轻，通常把形式主语it放在句首，真正主语搁置于句末
（1）It is certain that he will win the match.
（2）It is still a mystery what caused the accident.
（3）It is said that he has gone to shanghai.(=He is said to have gone to shanghai)
4. 宾语从句
4.1 定义
置于动词、介词等词性后面起宾语作用的从句叫宾语从句。宾语从句的语序必须是陈述语序。谓语动词、介词、动词不定式，v.-ing形式后面都能带宾语从句。有些形容词（afraid, sure等）之后也可以带宾语从句。
4.2 引导词
在复合句中作主句的宾语，引导词有：
连词：that (that可省略),whether,if
代词：who, whose,what,which
副词：when ,where, how, why
4.3 例句（斜体部分为从句）
1. The boy believes that he will travelthrough space to other planets.
2. I wonder whether(if) they will come to our party.
3. I’m thinking of whether we should go to see the film.
4. Can you tell me whom you are waiting for?
5. None of us knows where these new parts can be bought.
5. 表语从句
5.1. 表语定义
be动词或系动词(be, become, appear, seem, look, sound, feel, get, smell)+表语
The task is to remember as much knowledge as possible.
The sky seems cloudy and foggy.
系动词分类：
1. 五大感官动词look, sound, feel, taste, smell;
2. 状态系动词be, seem, appear, keep, remain, stay, prove;
3. 动态系动词get, fall, grow, turn, go, become, come, run, make;
4. 双谓语系动词rise, lie, die, etc.
Lei Feng died young.
The sun rose red.
5.2. 表从定义：用一个句子作为表语。（斜体部分为从句）
My suggestion is that you should do intensive study in grammar.
5.3. 表语从句连接词（斜体部分为从句）
5.3.1 从属连词：
that不作成分；whether, as, as if, as though。
The reason is that the cost isn’t high.
The question is whether you can do it on your own.
5.3.2 关系代词what, who, whom, whose, which, whoever, whichever, whatever
His worry is whatever has been stolen.
5.3.3 关系副词when, where, how, why,
This is where they once lived.
6. 同位语从句
6.1 定义：同位语从句描述的信息或内容等于先行词提到的信息或内容。
The fact that Daliang-Ronggui will be one of the administrative centers of Foshan is known to many citizens.
fact就是Daliang-Ronggui will be one of the administrative centers of Foshan.
6.2 引导词（斜体部分为从句）
that, whether, who, whom, whose, which, what; how, when, where, why.
that/whether:
We are astonished by the news that the housing price has grown by 100% in 2016.
We have some doubt whether they can complete the task on time.
关系代词:
I have no idea what he is doing now.
The question who should do the work requires consideration.
关系副词：
The question where the new commercial building should be built is still under discussion.
The report why the teacher was disqualified from the post is shocking.
6.3 常用先行词
advice, announcement, argument, belief, claim, conclusion, decision, evidence, explanation, fact, feeling, hope, idea, impression, information, knowledge, message, news, opinion, order, probability, promise, proposal, remark, reply, report, saying, statement, suggestion, thought, treat, warning, wish, word
希望以上长文能更加清晰地清楚大家对于名词性从句的疑问，让大家高效快速掌握。