定語從句（Attributive Clauses），在句中作定語，其作用是修飾主句的某個名詞性成分，相當于形容詞，所以又稱為形容詞性從句。通常定語從句皆置于它所修飾的名詞或代詞之后，被修飾的名詞或代詞叫做先行詞（antecedent），而引導定語從句的詞稱為關聯(lián)詞。

   朗閣海外考試研究中心分析認為，關聯(lián)詞常有3個作用：連接作用，引導定語從句；代替先行詞；在定語從句中擔當一個成分。關聯(lián)詞為關系代詞和關系副詞。關系代詞在定語從句中可以用作主語、賓語、定語等；關系副詞在定語從句中只用作狀語。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是關系代詞which引導的定語從句，用以修飾which的先行詞factor, which在從句中用作主語）

   ★ 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是關系副詞where引導的定語從句，用以修飾where的先行詞regions, where在從句中用作地點狀語）

   定語從句一般緊跟在其先行詞之后。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.（關系代詞that引導的定語從句緊跟其先行詞artificial irrigation systems之后）

   有時也可以與先行詞分離。如：

   ★ A new teacher will come tomorrow, who will teach you Chinese.（關系代詞who引導的定語從句與其先行詞teacher分離）

   用作關聯(lián)詞的關系代詞有that, which, who, whom, whose, as等，但朗閣海外考試研究中心在此提醒考生們絕對沒有“what”。who, whom, whose指人，who是主格，在定語從句中用作主語（在非正式英語中也可以用作賓語，并且用作賓語時可以省去不用）；whom是賓格，在定語從句中用作賓語（也可以省去不用）；whose是屬格，在定語從句中用作定語（有時也可以指物）。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.（主格關系代詞who在從句中作主語）

   ★ The man who he talked about is a professor.（在非正式英語中who代替了whom,也可以省去不用)

   ★ He doesn’t like the girl whom you refer to.（賓格關系代詞whom在從句中用作賓語，也可以省去不用）

    ★ The children whose parents give priority to work may feel lonely and alienated.（屬格關系代詞whose在定語從句中用作定語，指人）

   ★ The car whose wheels have been stolen was bought yesterday.（屬格關系代詞whose在定語從句中用作定語，指物，即car，可以用of which代替，但后者較為正式）

   That在定語從句中既可以用作主語，也可以用作賓語（在非正式文體中可省去）；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.（關系代詞that在定語從句中用作主語，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.（關系代詞that在定語從句中用作賓語，指物）

   ★ The students that hold a foreign degree seem to be more confident and independent.（關系代詞that在定語從句中用作主語，指人）

   which在定語從句中既可以用作主語，也可以用作賓語；一般皆指物（在非正式文體中可省去）。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.（關系代詞which在定語從句中用作主語）

   ★ Those surveyed had experienced chronic illnesses, for which orthodox medicine had been able to provide little relief.（關系代詞which在定語從句中用作賓語，可以省去）

   which在從句中也可以被用作定語和表語。如：

   ★ His money had been stolen, which news annoyed him.（關系代詞which在定語從句中用作定語)

   ★ They were completely mistrusted, which in fact, they were.（關系代詞which在定語從句中用作表語）

   關系代詞在從句中用作介詞賓語時，介詞可以位于從句之首，也可以位于從句之末。但以位于從句之首較為正式。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