名词性从句之主语从句

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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.