一 定义：在复合句中修饰名词或代词的句子被称为定语从句。在复合句中相当于一个形容词。
二 先行词：被定语从句修饰的名词或者代词叫做先行词。
三 关系词：引导定语从句的连词叫做关系词，分为关系代词和关系副词。
四 关系代词：
which（在定语从句中充当主语或宾语，偶尔也有定语，指物），
that（在定语从句中充当主语或宾语，可指人或者物），
who（在定语从句中充当主语或者宾语，指人），
whom（在定语从句中专门做宾语，指人），
whose（在定语从句中做定语，意思是“…的”），
as（在定语从句中充当主语或宾语，一般指事物，指代句子中的某一句话）。
定语从句缺少主语或宾语时，一般从关系代词里面根据先行词指人还是指物，挑选一个合适的。
which和as的区别：as引导的定语从句位置灵活，可放句首句中句尾，which引导的定语从句不可放在句首。as引导定语从句是意为“正如”，which没有明确意思。二者的共同之处在于都可以指代文中的一句话或者一件事。
as常用于以下结构中：as is/was +过去分词，as在该从句中充当主语；as+主语+谓语（及物动词），as在该从句中充当宾语。
五 关系副词：
when（在定语从句中做状语，指时间）
where（在定语从句中做状语，指具体的或者抽象的地点）
why（在定语从句中做状语，表原因，先行词常是reason）
当定语从句不缺主语或者宾语时，一般从三个关系副词里面根据先行词是表示时间、地点还有原因挑选一个合适的。
六 定语从句的分类
一般分为限制性定语从句和非限制性定语从句。简略而言，如果从句和主句之间用逗号隔开了，就算作非限制性定语从句，如果没有用逗号隔开，则可以认为是限制性定语从句（这种说法虽然不科学，但是对付高中试题，绰绰有余），主要看的是从句与主句只见关系是否密切。
He died, which made his parents sad.
The girl who stands behind a woman is beautiful.
定语从句一般还可以分为以下四种：
关系代词引导的定语从句
关系副词引导的定语从句
介词+which/whom引导的定语从句
名词/数词/分数/百分数/不定代词/形容词最高级/序数词等+of+which/whom引导的定语从句
He who studies hard will most likely succeed.
The house which he lives in belongs to his mother.
I will never forget the days I spent with my mother.
The things and persons that i can never forget have great influences on me.
The reason why he was late was that his mother was badly ill recently.
The house where he lives belongs to his mother.
I will never forget the days when I was with my mother.
He met me in 1995, when his father died.
The girl with whom he was talking was his sister.
The window through which we can see the playground was broken by someone.
I planted five trees in my garden, 3 of which died.
We have 50 students in our class, most of whom are girls.
There are some girls standing there, the most beautiful of whom is the girl in red.