It's very kind of you to help me.

To help me is very kind of you. 错误

You are very kind to help me. 正确

由此可知，it为形式主语，但真正主语并不是不定式，而是you. 由于代词作主语不能外置，所以由介词of引出，介词短语成了状语。

这种情况类似于双宾语和单宾语结构之间的转换：

give sb sth. sb为间接宾语。

give sth to sb. 语义上sb是间接宾语，但语法上to sb介词短语没有资格作宾语，只能分析为状语。