Who, whom和whose可用于非限制性定語從句中，代表某人或某人的，此時不能用that。如：

My sister, who is a nurse, came home for a few days.

我姐姐回家住了幾天，她是個護士。

which

which代表物，在從句中可作主語或賓語，作賓語時可以省略，但作介詞賓語且介詞位于其前時不能省略。例如：

The book which helps you most are those which make you think most.

最能使你獲益的書是那些最能讓你深思的書。

This is a factor which we must not neglect.

這是一個我們絕不能忽略的因素。

Where is the book from which you quoted this sentence?

你引用這句話的那本書在哪兒?

which可用于非限制性定語從句中，代表物。如：

The book, which I bought second-hand, is made of oak.