此句中it 的用法

题目

此句中it 的用法
It seemed as if he couldn't think of the right word.
此句中it是形式主语吗?我觉得不是,因为如果是真正的主语是后面的从句的话,那把这个从句代回原句也不通顺：as if he couldn't think of the right word seemed.
然而其他形式主语的句子真正主语代回原句都是通顺的.如It is not a good habit to stay up late.就可理解成：To stay up late is not a good habit
所以,那上句中it是什么呢?

答案

It在这里就是一个固定用法,It做形式主语有三种情况如下：1.代替不定式短语,Is it necessary (to complete the design before National Day?)2.代替动词-ing It's dangerous (walking on the ice).3.代替主语从句,It'...