定语是修饰名词或代词的句子成分；用一个完整的句子充当定语称为定语从句。所谓“完整的句子”的基本条件是必须具有主语和谓语。
定语从句位于它所修饰的名词、代词之后。
【专用术语】
先行词：被定语从句所修饰的名词或代词叫做先行词。
关系代词：which ；that ； who ； whom ；whose；as
关系副词：when ； where ； why
先行词、关系代词、关系副词都是定语从句的专用术语。关系代词和关系副词初看起来似乎同引起其他从句的which 、that、when等连词一样，其实它们在定语从句中有特殊的语法作用。
【关键知识】
关系代词和关系副词在定语从句中的语法作用：
1．代表先行词的意思。定语从句修饰哪个词，关系代词或关系副词就是那个词的意思。
2．在定语从句中充当句子成分。关系代词which、that、who在定语从句中可充当主语或宾语，whom 只能充当宾语，whose 充当定语；关系副词when 在定语从句中充当时间状语，where充当地点状语，why充当原因状语。
充分理解关系代词和关系副词在定语从句中的语法作用是学好定语从句的关键。
例1：With the pay that he received and saved, he went on with his studies at university.
关系代词that除引起定语从句he received and saved外，其重要的语法作用是代表先行词pay的意思，并在定语从句中充当received和saved的宾语。如果我们只看定语从句，其实就是he received and saved the pay,只是宾语the pay由关系代词that所表示。这正是定语从句的难点所在，关系代词的语法作用不能在句子中直观地看到，只能借助相关知识加以理解。
例2：By 1919, scientists who had been watching the stars supported his work and he quickly became world-famous.
关系代词who代表先行词scientists的意思，在定语从句中充当句子的主语。应该能够看出，定语从句实际是scientists had been watching the stars,主语scientist由关系代词who所代表。
例3：The student whose globe I borrowed did not come to school today.
whose在定语从句中充当globe的定语,它在代表先行词的意思时，与which、that、 who、whom的情况略有不同。whose的实际意思等于“the student’s”或“his”。whose globe在定语从句中作borrowed的宾语，修饰the student的定语从句为I borrowed the student’s globe。
例4：The train had divided in the station where it had stopped.
关系副词where在代表先行词的意思时等于“at the station”或“there”。如果只看定语从句，则是it had stopped at the station，显而易见at the station 是从句的地点状语，所以where 被称为关系副词。
例5：I can remember the time when it was difficult to make international phone calls.
关系副词when 的实际意思是“at that time”或“then”,定语从句为it was difficult to make international calls at that time(then)，when所代表的at that time(then)在定语从句中作时间状语。
例6：I don’t believe the reason why he is often late for class.
从例4、例5、例6中我们可以看出，关系副词where、 when由于在定语从句中充当时间或地点状语，因此我们在理解它们代表先行词的含义时，应当加入一个与其意思相配合的介词。
【相关知识及其运用】
1. 系代词which 和that 的区别
关系代词which只能指代“事、物”，不能指代“人”；that既能指代“人”，也能指代“事、物”。它们在定语从句中可以充当主语或宾语。除此之外，在具体的语言环境中，它们还有一些细微的差别。
下列情况只能用which,不能用 that：
① 在非限制性定语从句中只能用which。
A satellite is an object, either natural or man-made, which travels in an orbit round another object in space.
Which引起的是非限制性定语从句，只对先行词object做补充说明，没有限定先行词词义范围的作用。在书写形式上，非限制性定语从句和先行词之间用逗号隔开。
② 关系代词充当介词的宾语，尤其是当介词前置到先行词之后时，只能用which。
There is a rocket motor on the satellite by which the direction of the satellite can be changed if necessary.
如果我们只看定语从句并且将其恢复为正常的句子顺序，当然更容易理解：the direction of the satellite can be changed by a rocket motor (by which) if necessary。
介词连同关系代词which一起前置到先行词之后是极为常见的语言现象。理解这类句子的关键在于抓住介词在定语从句中的正常位置。
The old Olympic Games from which the modern games came began around the years 776 BC in Greece.
定语从句：…the modern games came from the old Olympic Games…（from which）
One subject to which country music often returns is “the good old days”.
定语从句：…country music often returns to one subject…(to which)
下列情况只能用that，不能用which:
① 当先行词是不定代词时。
I’m sure there is nothing that a woman can’t do.
that a woman can’t do是修饰不定代词nothing的定语从句，that 不能用which 代替。
下列试题专门考查这一知识点。
Finally, the thief handed everything ______ he had stolen to the police.
A. which B. what C. whatever D. that
被定语从句所修饰的先行词为不定代词everything，正确答案是D。
② 当先行词受到形容词最高级、序数词、the only等修饰时，只能用that。以下试题的意图便是考查此项内容。
The second book _____ I want to read is Travel in China.
A. which B. what C. that D. as
The only language _____ is easy to learn is the mother language.
A. which B. whose C. that D. it
Guilin is the most beautiful place _____ people all over the world want to visit.
A. where B. which C. what D. that
以上三题的答案均为that。
2. all 定语从句同what 引起的名词性从句的区别。
在语言实践中，all (that) I need的意思等于what I need，但两种表达方式的句子结构不同。all (that) I need中all是先行词，（that） I need是修饰all的定语从句，that代表all的含义，并在定语从句中充当宾语，所以可以省略。
what I need 是名词性从句，可以充当主语从句、表语从句、宾语从句。what的作用相当于the thing which，故what I need 的意思为the thing which I need，which是引起定语从句的关系代词。由于两种表达方式的意思相同，句子结构上又有相似之处，因此容易混淆。
以下两道高考试题专门考查这一知识。
All______ is needed is a supply of oil.
A. the thing B. that C. what D. which
答案为B。that代表不定代词all的含义，在定语从句中充当主语，不能省略，也不能用which代替。
To get the job started, ______ I need is your permission.
A. only what B. all what
C. all that D. only that
答案为C。
3. as引起的定语从句。
可以把as称作特殊的关系代词。这是因为，as在引起定语从句时其语法作用除了代表整个句子的含义和充当定语从句的主语、宾语外，它还有自己的特殊含义---- “像……那样”。
_____ is known to all, China will be an _____ and powerful country in 20 or 30 years’ time.
A. That; advancing B. This; advanced
C. As; advanced D. It; advancing
汉语译文：像大家所知道的那样，中国将在二、三十年后成为一个先进的强大的国家。
答案为 C。关系代词as代表China will be an advanced and powerful country in 20 or 30 years’ time全句的意思；在定语从句中充当主语；同时还有“像……那样”的含义。
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