为什么how do you say it was good to see someone这句要用it不用you?it不是它吗?应该不代表人吧..

首先分析一下句子：
It was good to see someone.这个可以作为一个完整的句子,it 代指 to see someone,这个句子可以改写为：To see someone is(was) good 动词不定式做主语.为了发出疑问,所以用how do you say 接上面的句子作为疑问句.
而且it is +adj(形容词)+to do something 是个很常用的结构,应该熟记
e.g.It is (really) happy to talk with my teacher.