在复合句中，修饰某一名词或代词的从句叫做定语从句。定语从句属于形容词性从句。
定语从句所修饰的词 语法上叫先行词。 定语从句放在先行词后面，由关系代词that、which、who、 which、 whose和关系副词where 、when，引导从句。对中学生来说，定语从句似乎是英语语法中最难的存在。因为关系代词和关系副词一方面要引导定语从句，另一方面它们还指代先行词在定语从句中充当主语，宾语，状语或定语中的某一句子成分。这是掌握定语从句的关键。
所以在定语从句中要选择相应的关系代词和关系副词，就要看它指代的先行词在从句中充当什么成分：
1）做主语或宾语，用that, which, who, whom
如果先行词是人，在从句中做主语或宾语，可以用who/whom /that。
如果先行词是物，在从句中做主语或宾语，可以用which/that。
做主语时，关系词不可省略，做宾语时可以省略。请看：
The man who is making the speech is a scientist from the USA.
在演讲的人是来自美国的科学家。
The doctor (who/whom /that) I saw yesterday was very kind.
我昨天见到的医生很和蔼。
The weather that we had this summer was beautiful.
我们这个夏天的天气很好。
Some pages in the book (that) I bought last week is missing.
我上周买的书中有几页不见了。
注意：如果先行词是不定代词，要用that指代：
Is there anything (that) we can do for you?
我们能为你做些什么吗？
Everything (that) he says seems silly to me.
他说的每一句话在我看来都很愚蠢。
All (that) you need is a good night's sleep.
= What you need is a good night's sleep.
你只需要睡个好觉。
意思相同，但句型结构不同。
All是主语，(that) you need是定语从句修饰它；
What you need是主语从句。
大家能明白吗？
先行词有形容词最高级，序数词以及any, very, only, all, no, just 等所修饰，要用that指代：
It is the best film (that) I've ever seen.
这是我看过的最好的电影。
The only thing (that) he cares is money.
他唯一关心的是钱。
The second book that was written two years later was a great success.
两年后写的第二本书非常成功。
现代口语里that常常代替关系副词，取代when, where, why和"介词+ which"引导定语从句。
这也说明语言是发展的，语法会有不断简化的过程。The family moved to Beijing the year（that / when / in which） he was born.
他出生那年全家搬到了北京。
He has no idea about the village（that / where / in which） he lived in his childhood.
他对小时候住过的村庄没有印象了。
非限制性定语从句中，先行词为物体，用which，而不用that
The dog, which had been with the family for 7 years, died last week.
陪伴了他们7年的狗上周去世了。
非限制性定语从句中，which还可以指代整个句子
Chris left without a word, which is quite a surprise.
克里斯一声不吭地走了，真让人吃惊。
which指代Chris left without a word
They all blamed it on me, which isn't what I'd expected.
他们都怪我，这我想也没想到。
which指代They all blamed it on me
2）做地点状语，用where或介词+which
This is the very house where/in which the great musician was born.
= This is the very house (which/that) the great musician was born in.
which/that 指代house做介词in的宾语
He has just come back from London, where he had worked for twenty years.
他刚从伦敦回来，他以前在那里工作了二十年。
This is the house which was built a hundred years ago.
这是一百年前建造的房子。
3）做时间状语，用when或介词+which
There are times when (at which) you need some help.
有时你需要帮助。
I'll never forget the days when (in which) we spent together with you.
我永远不会忘记和你在一起的日子。
注意，地点名词和时间名词，不一定做状语，也常常做主语，宾语等
The Great wall which/that was biult thousands of years ago attracts numerous tourists every year.
长城修建于几千年前，每年吸引着无数的游客。
I'd like to go to the island which/that I visited last year.
我想去我去年去过的那个岛。
4）做原因状语 用why或for which
It can't be the reason why he quit his job.
这不可能是他辞职的原因。
Is this the reason why（for which）you don't like the book?
这就是你不喜欢这本书的原因吗？
5）做定语，用whose或of which:
They live in a house whose walls/ the walls of which are red.
他们住在一所红墙房子里。
Yesterday I met an old friend, whose hair has already turned grey.
昨天我遇到一位老朋友，他已经两鬓苍苍。
Please pass me the book whose (of which) cover is green.
请把封面是绿色的那本书递给我。
定语从句可以说在英语里是作用最大的。它不仅仅限定修饰，也可以解释，说明，表示条件，原因等等。