原因：
形式主语it
作为形式主语的it并无实际语义,只是为满足语法上的需要,避免句子头重脚轻,它代替
的是句子的逻辑主语,概括起来会有如下情况：
句子的逻辑主语为不定式
如：It is very kind of you to give me a hand in time.
It is my pleasure to address the meeting.
It was pretty hard for him to bring up the child on his own.
真正的主语是后面的不定式短语