5.定语从句
定语从句是在句子中起形容词作用的主谓结构，通常修饰它前面的名词或代词，即它的先行词。定语从句是由关系代词和关系副词引导的。
1.关系代词引导的定语从句
(1)引导定语从句的关系代词有：who，whom，whose，which，that.
(2)关系代词在从句作主语、宾语、定语和表语的作用；当作宾语时，关系代词常常可省略。如：
This is the girl （whom） you‘ve been looking for.
(3)当定语从句所修饰的先行词是人或人格化的动物时，引导词用who，whom或whose;
如果先行词是无生命的东西，用关系代词which;而that可用于以上两种情况。
The man who robbed you has been arrested. That is the book which cost me twenty dollars. Which还可以指婴儿、动物和表示单数意义的集合名词，如：
The baby （which） the nurse brought in was Mary‘s child.(4)关系代词that和which的语法区别当先行词的前面有形容词的最高级修饰时，通常用that而不用which.如：
The final match is one of the most exciting football games that I have ever seen.当定语从句所修饰的先行词的前面有all，any，little，only，much等修饰时，或者这些先行词本身就是all，one，little，much，nothing等不定代词时，通常用that而不用which. I am interested in all that you have told me. It seems that is the only thing that we can do now.
当定语从句作介词的宾语时，只能使用which而不用that. The world in which we live is in constant motion and in constant change.当定语从句是与代词、数词或名词词组连用引导非限定性定语从句时，只能使用which而不用that. I bought a dozen eggs，five of which broke when I dropped the box at my door.
当先行词的前面有the only （first，last，same，next，very）等词修饰时，通常用that而不用which. This is the very movie that I want to see.当被用来指代整个句子，引导非限定性定语从句时，只能使用which而不是that. Things then improved，which surprises me. Her brother snatched the letter away，at which she was furious.
2. 关系副词引导的定语从句
(1)引导定语从句的关系副词有：when，where，why，how等。
(2)关系副词when，where，why在从句中作状语时，可用“介词 which”来替代。如：
The reason why （= for which） he is in hospital hasn‘t been known yet.
3. 限定性定语从句和非限定性定语从句
限定性定语从句与先行词关系密切，用来描述主句所涉及的人或物的具体情况，与主句不可分割。这种从句在口语中前后没有停顿，在文字中前后没有逗号。
非限制性定语从句与先行词之间的关系比较松散，为主句所描绘的人或物提供一些附加情况，并非绝对必要。这种从句在口语中有停顿，在文字中往往用逗号与主语隔开。
另外，非限定性定语从句可用which，who，whose，when，where等来引导，但不可以用that来引导。如：
The wine，which was in the cellar，was ruined.（非限定性）
The wine which （that） was in the cellar was all ruined.（限定性）
在限定性定语从句中，关系代词有时可省略；但是在非限定性定语从句中，关系代词不能省略。