主语从句有个特殊的用法，即it可作形式主语，一般用在主语较复杂的情况下，另外，主语从句在几种情况下不可以放在句首，下面，小编将给大家一一讲解相关的语法知识，一起来看看吧！
用it 作形式主语的结构
(1) It is +名词+that从句
It is a fact that … 事实是……
It is an honor that …非常荣幸
It is common knowledge that …是常识
(2) it is +形容词+that从句
It is natural that… 很自然……
It is strange that… 奇怪的是……
(3) it +不及物动词+that从句
It seems that… 似乎……
It happened that… 碰巧……
(4) it is+过去分词+that从句
It is reported that… 据报道……
It has been proved that… 已证实……
主语从句不可位于句首的五种情况
(1) if引导的主语从句不可居于复合句句首。
(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?
这五种情况，大家在今后的学习中要时刻注意，多思考才能不犯同样的错误。赶快收藏起来吧~