(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.
if可以引導(dǎo)主語從句，但必須放置句尾。
(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.
(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.
(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.
(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?
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