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：
a.连词：that, whether, if（这三个词都不作从句的成分，同时，that无含义，而whether和if都表“是否”）
b.疑问代词：who, whom, whose, what, which
c.疑问副词：when, where, why, how
（一）主语从句
  在复合句中充当主语的从句。
引导主语从句
   that引导主语从句时不同于其他的连词，既无词义也不作成分，只起单纯的连接作用，且通常不可以省。为了强调that引导的主语从句而置之于句首时，that不可省。
   ①That the driver could not control his car was obvious.
   ②That she was chosen made us very happy.
   ③That he will come is certain.
   ④That he would take the risk is true.
   ⑤That he should have married her isn't surprising.
★通常，that引导主语从句时用it做形式主语，把从句置于句尾，此时that有时可省。例如：
   ①It was obvious that the driver could not control his car.
   ②It made us very happy that she was chosen.
   ③It is certain that he will come.
   ④It is true that he would take the risk.
   ⑤It isn't surprising (that) he should have married her.
★如果以that从句为主语的句子是疑问句，就只能用先行词it结构。例如：
   ①Is it certain that he will come?
   ②Is it true that he would take the risk?
▲it做形式主语有以下几种不同的结构：
(1)it + be + 形容词+ that-从句
 ①It is likely that he will come.
 ②It is strange that she has ever trusted him.
 ③It is important that he（should) attend the meeting.
 ④It is best that he (should) go.
   此句型在表示惊讶、喜悦、遗憾等感情色彩时，that从句中的谓语动词也常用（should）+ v原形。
    It is strange that no one should have objected to the plan.
(2)it + be +名词+ that-从句
     It is common knowledge that the whale is not a fish...
     It's a pity that he should have massed the train.
(3) it + be + -ed分词 + that从句
     It is said that he is a famous writer.
     It is reported that there will be a storm this afternoon.
▲在表示建议、要求、命令等意义的被动结构中，that从句中的谓语动词常用（should）do。（参见虚拟语气部分）
    It is suggested that the meeting(should）be held this afternoon.
    It is demanded that he (should) leave at once.
(4) it + seem/happen/appear等不及物动词+that从句。（look不接that从句，它接to be结构或形容词）
     It seems that he is wrong.
     It appears that they are in urgent need of help.
     He looks (to be) surprised/happy/sad.
2. whether引导主语从句，意为“是否”，只起连接作用，不充当任何成分。(置于句首时必须用whether引导，置于句尾时，间或可用if。)
    Whether he will win the game is not clear.
    Whether he will come is uncertain.
    Whether she comes or not doesn't concern me.
    It is not clear whether/if he will come.
    It was uncertain whether he would come.