(1) if 引导的主语从句不可居于复合句句首。
　　It is uncertain if he will leave for Beijing tomorrow.
　　if可以引导主语从句，但必须放置句尾。
(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.
(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.
(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.
(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?

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