Paul is friends with Bill.为什么系动词用is

问题描述:

Paul is friends with Bill.为什么系动词用is
最近在学语法,从初级开始的~

paul是单数,因此用is,但后面有点问题,应该用单数,如果是复数,就要用of
也就是:Paul is a friend with Bill
或者是:Paul is one of the friends with Bill