Who, whom和whose可用于非限制性定语从句中，代表某人或某人的，此时不能用that。如：
My sister, who is a nurse, came home for a few days.
我姐姐回家住了几天，她是个护士。
which代表物，在从句中可作主语或宾语，作宾语时可以省略，但作介词宾语且介词位于其前时不能省略。例如：
The book which helps you most are those which make you think most.
最能使你获益的书是那些最能让你深思的书。
This is a factor which we must not neglect.
这是一个我们绝不能忽略的因素。
Where is the book from which you quoted this sentence?
你引用这句话的那本书在哪儿?
which可用于非限制性定语从句中，代表物。如：
The book, which I bought second-hand, is made of oak.