在復(fù)合句子中起名詞作用的句子叫名詞性從句。簡單把它理解成名詞，但形式是一個(gè)句子。那主語從句就是在句中作主語，主語從句的位置與陳述句中主語位置相同的，統(tǒng)觀主語從句的考核占有相當(dāng)比重，下面和具體來看一下主語從句不可位于句首的五種情況吧！
(1) if 引導(dǎo)的主語從句不可居于復(fù)合句句首。
It is uncertain if he will leave for Beijing tomorrow.
(2) It is said , (reported) …結(jié)構(gòu)中的主語從句不可提前。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… 結(jié)構(gòu)中的主語從句不可提前。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 …結(jié)構(gòu)中的主語從句不可提前。例如：
It doesn’t matter whether he is wrong or not. (right)
Whether he is wrong or not doesn’t matter. (wrong)
(5) 含主語從句的復(fù)合句是疑問句時(shí)，主語從句不可提前。例如：
Is it likely that it will rain in the evening? (right)
Is that will rain in the evening likely? (wrong)
注：What 引導(dǎo)主語從句時(shí)在句時(shí)在從句中充當(dāng)句子成分，如主語．賓語．表語，而that 則不然。
例如： 1) What you said yesterday is right.
2) That she is still alive is a consolation.