详细讲一讲one of which

one of which (= 其中之一) 可以引导定语从句,而 one of them (= 其中一个) 不行,如：
I have five books,one of which is English.(=我有五本书,其中有一本是英语书.) ；
I have five books ,and one of them is English.(意思同上)；
one of which 只能指“物”；而 one of them 既能指“物”,也能指“人”,如：
There are five children in the room,and one of them is reading.(= 房间有五个孩子,其中一个在看书).

举两个例子吧~
She has two pens, one of which is red.
She has two pens, and one of them is red.
你可以看到，如果主句与从句中有连词and，那么后面就用them，如果没有，那么该句就是非限制性定语从句，就用one of which.

For example: I have many questions, one of which is about fishing island.
which代表前面名词,其数与名词一致, 相当于说one of the questions,中心词是one所以后面用is再如:
The boy finds some books, one of which was written by LuXun.