通常来说,说话出现了主语从句的情况下,为了避免主语冗杂而使用it 做形式主语
That he wakes up early every day is a good habit.
it is a good habit that he wakes up early everyday
不定式做主语
To study English is important for him
it is important for him to study English
还有很多,可是基本上就是这么种形式
英语中一般什么情况用it is作形式主语开头的句型