名词性从句，即指性质相当于名词的从句，它包括主语从句、宾语从句、表语从句和同位语从句。在句子中起名词的作用。

在近年的英语考试中，名词性从句考得最多的是主语从句、宾语从句、表语从句。

主语从句，即在复杂句中充当主语成分的句子。主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。

引导主语从句的连词主要有：
从属连词：that whether
连接代词：who whoever whom whose what whatever which whichever
连接副词：when where how why

今天和小编一起来看下主语从句不可位于句首的五种情况。

(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.

例如： 
It is said that President Jingo will visit our school next week. (right) 
That President Jiang will visit our school next week is said. (wrong) 

(3) It happens…, It occurs… 结构中的主语从句不可提前.

例如： 
It occurred to him that he failed in the examination. (right) 
That he failed in the examination occurred to him. (wrong) 

(4) It doesn’t matter how/whether …结构中的主语从句不可提前.

例如： 
It doesn’t matter whether he is wrong or not. (right) 
Whether he is wrong or not doesn’t matter. (wrong) 

(5) 含主语从句的复合句是疑问句时,主语从句不可提前.

例如： 
Is it likely that it will rain in the evening? (right) 
Is that will rain in the evening likely? (wrong) 

好了，以上的这些还请大家务必要掌握，在日常的学习中一定要勤加练习，在具体的句子中加以理解，一切问题就可以迎刃而解了。