nice to see you这里to的作用?

这句省略了 It is
It is nice to see you.
to say you是主语to see you.不是宾语吗？为什么是主语呢？这句话其实是 To see you is nice .但这样头重脚轻 所以用it 当形式主语，真正主语是 to see you.