名词性从句： 包括主语从句、表语从句、宾语从句和同位语从句。
引导名词性从句的连词有：who; whom; whose; what; which ;when ;where; why ;how; that; if ;whether; whatever; whenever ; wherever; however 等。
名词性从句注意事项：
从句的语序为陈述句；
连词that不充当任何成分；
what总是要充当主语、表语、宾语等。
If和whether 的区别： if引导宾语从句和条件状语从句； whether引导主语从句、表语从句、宾语从句和同位语从句.
（1） 主语从句： 用作主语的从句，叫主语从句。
主语从句可以直接放在句首；也可用it作形式主语，而将从句放在句末。
常见的用it作形式主语的复合结构：
It is +名词＋that 从句； It is a fact(a pity, a shame, no wonder, good news…)that…
It is+形容词＋that从句；It is necessary(strange, important, wonderful, possible , likely,…)that… 这类句中谓语动词多为（should）+动词原形。
It is +过去分词＋that 从句； It is said (reported, decided, believed,..)that…
It +不及物动词＋that 从句；It seems (happened, doesn’t matter , has turned out,…)that…
What you need is more practice.(what在从句中作need的宾语)
What is hard is to do good all one’s life and never do anything bad.（ what在从句中作主语）
What he will be in the future is known to us.（what在从句中作be的表语）
Who will go makes no difference.
How this happened is not clear.
Why he did that wasn’t quite clear.
Whether she will join us won’t make too much difference .(此处不能用if)
Whoever comes will be welcome.( whoever 在名词性从句中相当于anyone who )
Whatever she did was right. （whatever 在名词性从句中相当于anything that）
It is possible that I may not be able to come. （it是形式主语，真正的主语是划线部分）
It doesn’t seem likely that she will be here.
Isn’t it strange that he should have left without telling us?
It occurred to me that he had forgotten to take his notebook with him.
That we need more equipment is quite obvious.
Has it been announced when the planes are to take off?
(2) 表语从句： 用作表语的从句叫表语从句。 它位于连系动词（如be，seem，remain 等后）
The question is whether we should accept their invitation.
The city is no longer what it used to be.
This is why we put off the meeting. （why表结果）
That is because he was ill. (because表原因)
The reason for his absence was that he was ill.(此处不能用because代替that . 句型： The reason …is that…)
As if 引导的表语从句有时可用虚拟语气，表示可能性小。
He acted as if he hadn’t eaten for a long time.
表示建议，命令，要求一类的表语从句要用虚拟语气：
My suggestion is that you should practise reading English every day.
(3) 宾语从句： 作宾语的从句叫宾语从句。
作动词的宾语： He don’t know where the post office is.
在动词suggest, order, demand, require等表示建议，命令，要求的宾语从句要用虚拟语气 (宾语从句的谓语用should+动词原形， should 可省略。）
I suggested that you (should)start right now.
如果宾语从句后有宾语补足语，要用形式宾语it,而将从句放到补足语后面。
I thought it strange that he didn’t pass the exam.
在think, believe, suppose, expect等动词后的宾语从句中，如果从句谓语是否定含义，则不用否定形式，而将主句谓语动词think等变为否定形式。
I don’t think you are right. 我想你是不对的。
He doesn’t know where the post office is.
在think, believe, imagine, suppose, guess, hope 等动词以及I’m afraid等后，可用so代替一个肯定的宾语从句，还可用not代替一个否定的宾语从句：
Do you believe it will clear up?
I believe so . (I don’t believe so./ I believe not.)
作介词的宾语:
Did she say anything about how we should do the work?
That 引导的宾语从句只有在except, in, but, besides 等少数介词后偶尔可能用到。
Your article is good except that it is too long.
有时在介词和其宾语从句的中间加形式宾语it.
I’ll see to it that everything is ready. 我将负责把一切准备好。
作形容词的宾语：
I’m afraid that I have made a mistake.
当 if 和whether 引导宾语从句的区别，即用 whether不用if的情况：
作介词宾语
It depends on whether it is fine.
Whether +to do
I ‘m not sure whether to leave this afternoon.
. Whether or not
He can not decide whether or not take the exam.
作discuss 的宾语 We are discussing whether we will hold a meeting this weekend
doubt 用于肯定句中，宾语从句可以用if 或whether 引导。
I doubt if (whether)you will come for a visit tomorrow.
doubt 用于否定句或疑问句中，宾语从句用 that 引导。
He doesn’t doubt that his mother will give him a ride home for sure.
(4) 同位语从句：同位语从句跟在一个名词后（如fact, idea, news, hope ,belief, thought, doubt, proof, belief 等），对其作进一步解释。
I have no idea that you were here.
He cant answer the question how he got the money.
同位语从句和定语从句的区别：
从意义上讲，前者对名词加以解释说明，后者对名词进行修饰限定。
从结构上讲，前者由连接词引导，后者由关系词引导。
从内涵上讲，前者所说明的名词与从句没有逻辑关系，that不可省略。后者所限定的名词是从句逻辑上的主语、宾语、表语、定语、状语等。当被限定的名词是从句逻辑上的宾语时，that 可省略）
The news that they won the match is true. (同位语从句,news 和从句没有逻辑关系, that 不可省略)
The news that you told us yesterday is true. （ 定语从句，news 是told 的逻辑宾语,that可省略）
参考资料：http://study.zhupao.com/infoview/Article_4069.html