1. who
who在定語從句中擔任主語，其先行詞必須是表示人的名詞或代詞。who也可以代替在從句中擔任賓語的whom，但是它前面不能有介詞。
I have a friend who likes listening to classical music. （who在從句中作主語，代替前面的a friend）
Luckily none of the people who I know were killed in the earthquake. （who在從句中作賓語，代替前面的the people）
2. which
which所代替的先行詞是表示事物或動物的名詞或代詞，一般不用來指人。
① which在從句中作主語或賓語。
She was not on the train which arrived just now. （which在從句中作主語，代替先行詞the train）
The watch (which) you gave me keeps good time. （which在從句中作賓語，代替先行詞the watch，可省略）
② which引導的定語從句可表示一個概念。
which的先行詞有時不是一個詞，而是主句所表示的整個現象，此時，which前一定要使用逗號。
James did really well in his exams, which was a big surprise. （which相當於and that，其先行詞是前面整個句子）
He has to have lessons on Sundays, which he doesn"t like at all.
which在定語從句中有時也能作定語。這種用法比較正式，經常出現在書面語中。
The Beijing Olympic Games are being held in three years, by which time I will be 21.
It might snow this weekend, in which case we won"t go hiking in the mountains.
1. who
who在定語從句中擔任主語，其先行詞必須是表示人的名詞或代詞。who也可以代替在從句中擔任賓語的whom，但是它前面不能有介詞。
I have a friend who likes listening to classical music. （who在從句中作主語，代替前面的a friend）
Luckily none of the people who I know were killed in the earthquake. （who在從句中作賓語，代替前面的the people）
2. which
which所代替的先行詞是表示事物或動物的名詞或代詞，一般不用來指人。
① which在從句中作主語或賓語。
She was not on the train which arrived just now. （which在從句中作主語，代替先行詞the train）
The watch (which) you gave me keeps good time. （which在從句中作賓語，代替先行詞the watch，可省略）
② which引導的定語從句可表示一個概念。
which的先行詞有時不是一個詞，而是主句所表示的整個現象，此時，which前一定要使用逗號。
James did really well in his exams, which was a big surprise. （which相當於and that，其先行詞是前面整個句子）
He has to have lessons on Sundays, which he doesn"t like at all.
which在定語從句中有時也能作定語。這種用法比較正式，經常出現在書面語中。
The Beijing Olympic Games are being held in three years, by which time I will be 21.
It might snow this weekend, in which case we won"t go hiking in the mountains.