(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?