首先我們大家都明白在復(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，下面和小編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.?