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。
朗阁海外考试研究中心分析认为，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，但朗阁海外考试研究中心在此提醒考生们*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 ‘ap
