一、定語從句的定義
在複合句中， 修飾名詞或代詞的從句叫做定語從句。被定語從句所修飾的詞叫先行詞。引導定語從句的詞叫關係詞。關係詞包括關係代詞who(賓格whom,所有格whose),that, which 和關係副詞where, when, why 等。
二、定語從句的種類
定語從句可以分為限定性定語從句和非限定性定語從句。
1.限定性定語從句：它是先行詞在意義上不可缺少的定語，如果去掉，主句的意思就不完整或失去意義。
The man that wears a red hat is my uncle.
2.非限定性定語從句：它只是對先行詞作附加補充說明，如果去掉，主句的意思仍然清楚完整。書寫時往往用逗號分開。非限定性定語從句一般不用that 引導，而且不可以省略關係詞。
The old woman, who lives on her own, has a cat for company.
Beijing, which is the capital city of China, is a very beautiful city.
三、關係代詞的用法
在定語從句中， 關係代詞起著連接主句與從句，指代先行詞和在從句中作句子成分的三重作用。關係代詞的選用取決於主句中的先行詞以及關係代詞在從句中作何種成分。
（一）關係代詞who, whom 的用法
1.who 可以代替人（即它的先行詞必須是人），在從句中擔任主語，往往也可以代替在從句中擔任賓格的whom，但它的前面不能有介詞，如果帶介詞則必須用賓格whom，即“介詞+whom”.
Jim is a person who always has novel ideas.
The person who/whom you just talked to is Mr. Depp.
The person to whom you just talked is Mr. Depp.
（二）關係代詞whose 的用法
關係代詞whose 是關係代詞who 的所有格形式，它既可以代人，也可以代物。當whose 代物時，相當於of which。
Do you know Peter whose father happens to be working in your company?
The tourist wanted to book a room whose window faces south.
=The tourist wanted to book a room the window of which faces south.
=The tourist wanted to book a room . The room’s window faces south.
（三）關係代詞that, which 的用法
（1）在限定性定語從句中指代物時，which 和that 一般可通用。
The new house that/which I has just bought is about six miles away.
（2）限定性定語從句中只用that 而不用which 的情況。
A）先行詞有形容詞最高級修飾時
This is the best way that has been used against pollution.
B) 先行詞有序數詞修飾時
This is the third time that they have met.
C)主句已有疑問詞who 或which 時
Which of the cars that are in front of the hotel belongs to you?
D) 先行詞既有人又有物時
He talked about the schools and teachers that he had visited.
E) 先行詞為all, much, few, little, something, everything, anything, nothing, none 等不定代詞時
It’s easy to do the repair. All that you need is a hammer and some nails.
四、關係副詞的用法（where, when, why）
關係副詞在從句中均做狀語，所以具體選用什麼關係副詞，主要取決於關係副詞在從句中充當什麼類型的狀語。
（一）關係副詞where 的用法
由關係副詞where引導的定語從句，其先行詞主要是表示地點的名詞，如building, city, room等，where 在從句中作地點狀語。
The hotel where we stayed was very clean.= The hotel (that/which) we stayed at was very clean.
（二）關係副詞when 的用法
由when引導的定語從句，其先行詞主要是表示時間的名詞，如 time, day, date,等，when 在從句中作時間狀語。
I’ll never forget the time when we first met in London .
=I’ll never forget the time during which we first met in London.
（三）關係副詞why 的用法
由why 引導的定語從句，常用在先行詞reason 後面，why 在句中作原因狀語。
The reason why I got a job was that I worked hard.
五、關係代詞和關係副詞引導的定語從句的區別
如果關係詞在從句中製作主語或賓語，就是關係代詞；如果關係詞在定語從句中做狀語，就是關係副詞。例句比較：
This is the college (that/which) I visited.
This is the college where I studied three years ago.
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