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。
1.that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.