通俗地说，在口语中，形式主语是用it放在句首代替当主语的动词不定式，以避免头重脚轻，失去平衡：
It is difficult to answer this question. 这是个很难回答的问题。
=To answer your question is difficult.
动词不定式的逻辑主语是执行这个不定式动作的人：
It is difficult for me to answer this question. 要我来回答这个问题有困难。
=For me to answer your question is difficult.
There are still many questions for us to discuss.我们还有许多问题要讨论。
He is always the first (person) to come.他总是第一个来的。
It's foolish of him to believe what she said.他相信她说的话真愚蠢。