在学习名词性从句之前，我们首先要明白一个词：复合句
什么叫复合句
     由一个主句和一个或一个以上的从句构成的句子叫复合句。在复合句中主句是全句的主体，从句是全句的一个成分，不能独立。从句通常是用关联词来引导的。在这里关联词还起联系从句和主句的作用。 
一、从句的种类：
     主语从句、宾语从句、表语从句、同位语从句，定语从句和状语从句
二、名词性从句的连接词
1. 单纯连词
在从句中不补充当任何成分，只起到连接从句的功能。
that 没有实际意义，而whether/ if 在句子中表示“是否” 的意义。
Eg : That the prices of rice will go up is certain. (that引导主语从句，补充当任何成分)
       He asked if I could come to see him this weekend.
     形式主语it 代替主语从句位于句首时，常见的几种情况：
     ① It + adj +that 从句(大多数情况下that从句后用should+动词原形)
      Adj: necessary  right  clear  unlikely  strange  obvious  natural  important
      Eg: It is necessary to learn English well.
      ② It +系动词+名词+that 从句
      Non: a pity  an honor  good news  a wonder  a fact  no wonder  no surpeise
      Eg: It is a fact that you are a good boy.
      ③ It +be + 过去分词+that从句
      过去分词：said / heard/ reported/ told/ decided/suggested/thought/considered/ hoped/ announced 
      Eg: It is announced that the plan has been canceled.
     注意：
     It + be+ suggested/ advised/ ordered/ requested/ insisted/ demand+ that从句， that从句要用should +动词原形，should可以省略。
     Eg: It is suggested that you (should ) spend more time in studying English .
     当suggest表示“表明”时，不可以用虚拟语气
     Eg: It suggests that you are an honest boy.
     Insist 表示“坚持某种说法（事情已经发生过了）”，不用虚拟语气。
     表示“坚持要做（事情没有发生）”，要用虚拟语气。
     Eg: she insisted that I have never done such thing .
           She insisted that I study abroad.
     ④It +动词（+宾语或状语）+that 从句
     动词如： appear(显得)/ happen(碰巧)/ matter(重要)/ occur(出现)/ turn out(结果是)
     Eg: It happened to me that I was away when he came.主语从句不可位于句首的五种情况：
     注意：
     （1）if 引导的主语从句不可居于复合句句首。
     （2）It is said /reported?结构中的主语从句不可提前。
       正确表达：It is said that President Jiang will visit our school next week.
       错误表达：That President Jiang will visit our school next week is said.
     （3）It happens/occurs?结构中的主语从句不可提前。
       正确表达：It occurred to him that he failed in the examination.
      错误表达：That he failed in the examination occurred to him.
     （4）It doesn’t matter how/whether ?结构中的主语从句不可提前。
       正确表达：It doesn’t matter whether he is wrong or not.
       错误表达：Whether he is wrong or not doesn’t matter.
     （5）含主语从句的复合句是疑问句时，主语从句不可提前。
       正确表达：Is it likely that it will rain in the evening?
      错误表达：Is that will rain in the evening likely?