当名词前已经有修饰词,例如a,an,one,this,that,these,those,some,any,several,each,every,such,another等,物主代词又不能直接和这样的词一起前置,所以用双重所有格
公式为：a,an,one,this,that ...+名词+of +名词性物主代词
举下面的例子来说明你的问题：
a friend of my father's one book of mine this cup of his some classmates of Mike's
several friends of hers