题目内容：

关于名词所有格 名词+of+名词,后一名词为什么有时候要加(’s),有时候又不加?
例：the girl’s name
→the name of the girl
女孩的名字
my father’s friend
→the friend of my father’s
我父亲的朋友
请回答一下重点，〖我是说在已经用了of中的短语〗，后一名词为什么有时候要加(’s),有时候又不加？

优质解答

这本问题本来我也在纠结,不过现在我搞清楚了,我来回答你.刚看了书,把书上的内容搬过来给你,
Double genitive
The double genitive,also referred to as the "post genitive",is so called because it is a combination of the genitive and the of-phrase:
a friend of my brother's
some neighbours of my grandparents'
Note that the second noun in the double genitive construction almost always refers to persons,never to objects.
Also note that the first noun usually has indefinite reference (typically premodified by the indefinite article) and the second noun is always definite:
a friend of my brother's
some friends of my parents'
any friend of the doctor's
a lot of friends of that doctor's
Exceptions to the rule are the cases where the first noun is premodified by a demonstrative or postmodified by a relative clause:
that friend of my brother's
those ideas of your sister's
the friend of Bob's who was here last night