Tony is a friend of my brother's.

问题描述:

Tony is a friend of my brother's.
为什么有了of已经表示"的"了还要加'?
能不能说Tony is a friend of my brother.

二楼理解的完全正确.a friend of my brother's 即 a friend of my brother's friends(我哥哥朋友中的一个).再如:a friend of mine
其实这种格式在英语语法中叫“双宾格”