定语从句(Attributive Clauses)在句中作定语，其作用是修饰主句的某个名词性成分，相当于形容词，所以又称为形容词性从句。通常定语从句皆置于它所修饰的名词或代词之后，被修饰的名词或代词叫做先行词(antecedent)，而引导定语从句的词称为关联词。
关联词常有3个作用：连接作用，引导定语从句;代替先行词;在定语从句中担当一个成分。关联词为关系代词和关系副词。关系代词在定语从句中可以用作主语、宾语、定语等;关系副词在定语从句中只用作状语。如：
★ One factor which may help ensure that some endangered languages do not die out completely is people's increasing appreciation of their cultural identity.(which may help ensure that some endangered languages do not die out completely是关系代词which引导的定语从句，用以修饰which的先行词factor, which在从句中用作主语)
★ And even in regions where new projects seem warranted, we must find ways to meet demands with fewer resources, respecting ecological criteria and to a smaller budget.(where new projects seem warranted是关系副词where引导的定语从句，用以修饰where的先行词regions, where在从句中用作地点状语)
定语从句一般紧跟在其先行词之后。如：
★ Food production has kept pace with soaring populations mainly because of the expansion of artificial irrigation systems that make possible the growth of 40% of the world's food.(关系代词that引导的定语从句紧跟其先行词artificial irrigation systems之后)
有时也可以与先行词分离。如：
★ A new teacher will come tomorrow, who will teach you Chinese.(关系代词who引导的定语从句与其先行词teacher分离)
用作关联词的关系代词有that, which, who, whom, whose, as等，但在此提醒考生们绝对没有“what”。who, whom, whose指人，who是主格，在定语从句中用作主语(在非正式英语中也可以用作宾语，并且用作宾语时可以省去不用);whom是宾格，在定语从句中用作宾语(也可以省去不用);whose是属格，在定语从句中用作定语(有时也可以指物)。如：
★ The American zoologist Donald Griffin, who was largely responsible for the discovery of sonar in bats, coined the term ‘echolocation' to cover both sonar and radar, whether used by animals or by human instruments.(主格关系代词who在从句中作主语)
★ The man who he talked about is a professor.(在非正式英语中who代替了whom,也可以省去不用)
★ He doesn't like the girl whom you refer to.(宾格关系代词whom在从句中用作宾语，也可以省去不用)
★ The children whose parents give priority to work may feel lonely and alienated.(属格关系代词whose在定语从句中用作定语，指人)
★ The car whose wheels have been stolen was bought yesterday.(属格关系代词whose在定语从句中用作定语，指物，即car，可以用of which代替，但后者较为正式)
That在定语从句中既可以用作主语，也可以用作宾语(在非正式文体中可省去);既可以指人，也可以指物。如：
★ These methods include strength training that duplicate what they are doing in their running events as well as plyometrics, a technique pioneered in the former Soviet Union.(关系代词that在定语从句中用作主语，指物)
★ Put another way, basic heath-care is now recognized as a ‘public good', rather than a ‘private good' that one is expected to buy for oneself.(关系代词that在定语从句中用作宾语，指物)
★ The students that hold a foreign degree seem to be more confident and independent.(关系代词that在定语从句中用作主语，指人)
which在定语从句中既可以用作主语，也可以用作宾语;一般皆指物(在非正式文体中可省去)。如：
★ Eye position in freshwater dolphins, which often swim on their side or upside down while feeding, suggests that what vision they have stereoscopic forward and upward.(关系代词which在定语从句中用作主语)
★ Those surveyed had experienced chronic illnesses, for which orthodox medicine had been able to provide little relief.(关系代词which在定语从句中用作宾语，可以省去)
which在从句中也可以被用作定语和表语。如：
★ His money had been stolen, which news annoyed him.(关系代词which在定语从句中用作定语)
★ They were completely mistrusted, which in fact, they were.(关系代词which在定语从句中用作表语)
关系代词在从句中用作介词宾语时，介词可以位于从句之首，也可以位于从句之末。但以位于从句之首较为正式。如：
★ This has been encouraged through programs of language classes fro children and through ‘apprentice' scheme, in which the endangered language is used as the medium of instruction to teach people a traditional skill.(关系代词which在定语从句中用作介词in的宾语，介词位于定语从句之首，即which之前)
★ This has been encouraged through programs of language classes fro children and through ‘apprentice' scheme, which the endangered language is used as the medium of instruction to teach people a traditional skill in.(介词in位于定语从句之首，which在此可以省去)
像during, except或者表示分割作用的of等介词通常放在关系代词的前面。如：
★ The years during which he was away were long years to her.
★ He wrote many books, some of which you will read some day.
关系代词who和that用作介词宾语时，介词须置于句末。如：
★ The writer you referred to was an unknown writer.(关系代词who用作介词to的宾语，介词to须位于定语从句之末，who在口语中可以省去)
★ This is the problem that he has been complaining about.(关系代词that用作介词about的宾语，介词about须位于定语从句之末)
有时定语从句中还有其他成分，介词则位于定语从句之中。如：
This is the engineer who he cooperated with in the project.
先行词指人时，关系代词既可以用who, 也可以用that。但关系代词在定语从句中用作主语时，多用主格who。如：
★ Beyond the practical need to make order out of chaos, the rise of the dictionaries is associated with the rise of the English middle class, who were anxious to define and circumscribe the various world to conquer.(persons, those, people等先行词多用关系代词who)
★ It is Tom who should be punished.(在强调结构中，指人时多用who)
★ The American zoologist Donald Griffin, who was largely responsible for the discovery of sonar in bats, coined the term ‘echolocation' to cover both sonar and radar, whether used by animals or by human instruments.
在下列情况中多用that, 而不用who或者which。如：
★ much (很多)
little (很少)
none (没有任何)
everything (任何事情) +先行词 +that
everybody (任何人)
nothing (没有任何)
nobody (没有人)
E.g. There is nothing in the world that can frighten him.
★ 先行词前面有最高级形容词修饰时
He is the most boring speaker that I have ever heard.
This is the most interesting book that I have ever read.
★ 先行词有以下代词或者有下列代词修饰时
the first
the last
the only
the same +先行词 +that
the very
all
any
no
every
E.g. This is the last chance that you have.
This is the very thing that I lost yesterday.
This is the first thing that he wants to do.
You are the only friend that we have.
He is the last person that Tom wants to see.
He has told the police all that he knew.
★ 前面已经有疑问词who或者which时
E.g. Who that knows him will trust him?
Which of these trains is the one that goes to Nanjing?
Which is the bike that you have lost?
★ 兼指人和物/事物时
E.g. The man and the horse that fell into the river were drowned.
Look at the girl and her dog that are crossing the bridge.
在下列情况中则多用which。如：
★ 离先行词较远时
He told me the story of the craftsman which I narrated at the beginning of this book.
★ those + 复数名词 之后
You should buy one of those clothes which sell best.
★ which比较正式，非正式英语中也可以用that
Some water experts are now demanding that existing infrastructure be used in smarter ways rather than building new facilities, which is increasingly considered the option of last, not first, resort.
★ 描述性定语从句一般用which
Beijing, which was China's capital foe more than 800 years, is rich in cultural and historic relics.
as, than, but也可以用作关系代词。如：
★ The project was completed on time, as was expected before.(关系代词as在定语从句中用作主语，其先行词是前面的整个句子)
★ They have never met such things as he met.(关系代词as与指示代词连用，在定语从句中用作宾语，其先行词是such things)
★ Tourism, holidaymaking and travel are these days more significant social phenomena than most commentators have considered.(关系代词than在定语从句中用作宾语，其先行词是social phenomena)
★ There are very few but admire his talents.(关系代词but在从句中用作主语，其先行词是few, but= who don't)
用作关联词的关系副词有where, when, why等，when在定语从句中用作时间状语，其先行词是表时间的名词。如：
★ Just at the time when it became obvious that health-care resources could not possibly meet the demands being made upon them, people were demanding that their fundamental right to health-care be satisfied by the state.(关系副词when的先行词是the time)
since, before, after, as也可以用作表时间的关系副词。如：
★ At exactly the same time as this new realization of the finite character of health-care resources was sinking in, an awareness of a contrary kind was developing in western societies.(as用作关系副词)
★ Every day since she came has been most enjoyable.(since用作关系副词)
★ On the day before the war broke out he was born.(before用作关系副词)
★ The year after she left school she spent abroad.(after用作关系副词)
where在定语从句中用作地点状语，其先行词是表地点的名词。如：
★ I know the place where we can have a talk.
★ The zoo is an ideal research centre where scientists can experiment on animals to find out the causes of some disease and develop effective cures.
where的先行词可以是有地点含义的抽象名词。如：
★ He has reached a point where a change is needed.(where的先行词point是抽象名词)
why在定语从句中用作原因状语，其先行词只有reason。如：
★ That is the reason why he set up his own business.(why的先行词是reason)
★ The company refused to disclose the reason why he left.(why的先行词是reason)
有时候why可以省去。如：
★ The reason he came here is obvious.(reason后面省去why)
有时候why可以用that替代。如：
★ The reason that he failed was lack of patience.(why由that替代)
关系副词 = 介词 + which(关系代词)
where = in/at which; when = in/at which; why = for which
定语从句分为限制性定语从句和非限制性定语从句。考生注意，限制性定语从句与先行词关系密切，对它有限制性作用，因此不可缺少，如果省去，会使主句的意思不完整、不明确。限制性定语从句前面一般不用逗号。如：
★ The man who was driving the car was drunk.
★ The speech which was made by the Minister made his supporters delighted.
★ The police told us that he was the only person that was suspected.
★ This is the place where the three roads meet.
★ I shall never forget the day when she was born.
★ The reason why he was late was unknown.
★ The house whose windows are broken is Mr. A's.
非限制性定语从句也称为描述性定语从句。非限制性定语从句只与先行词有一种松散的修饰关系，在口语中用停顿的方法表示，在书面语中用逗号分开。因此从句中的关系代词不能省略。that一般不引导非限制性定语从句。如：
★ Water, which is a clear liquid, is the giver and taker of life.
★ They gave up the plan, which was a very good one.
