It 做形式主语 可以代替不定式 动名词 和主语从句
It is very difficult to learn English well.
It is worth reading more books
It is well known that Yuan Longping is a famous scientist
不定式作主语强调临时的动作,动名词作主语强调习惯
To give is better than to receive
Doing eye exercises is good for your health