在复合句中，作定语修饰名词、代词或句子的从句，叫做定语从句。它所修饰的名词、代词或句子叫做先行词；连接先行词与从句的词叫关系词，包括关系代词和关系副词。
定语从句-关系代词的用法
在复合句中，作定语修饰名词、代词或句子的从句，叫做定语从句。它所修饰的名词、代词或句子叫做先行词。根据定语从句与先行词的关系紧密程度不同，可将定语从句分为限制性定语从句和非限制性定语从句。
① 制性定语从句和非限制性定语从句
制性定语从句：
用法：对先行词起限制作用，是先行词不可缺少的定语，如果去掉，主句的意义就会不完整、不明确或失去意义；与主句关系密切，书写时不用逗号与主句隔开。
关系词：that、which、who、whom、whose、when、where、why、as
例句：People who take physical exercise live longer. 进行体育锻炼的人活得长些。
非限制性定语从句：
用法：只对先行词起补充说明的作用，和主句关系不是很密切，即使去掉，主句的意思仍然清晰；往往用逗号与主句隔开；不能用that引导，关系代词作宾语时也不能省略。
关系词：which、who、whom、whose、when、where、as
例句：His daughter, who is in Boston now, is coming home next week. 他女儿现在在波士顿，下星期回家。
② 关系代词引导的定语从句
关系代词代替前面的先行词，在定语从句中作主语、宾语或定语等，其用法具体如下：
which，指代物、句子，作主语、宾语。
例句：The work which has just been finished is very important.
that，指代人、物，作主语、宾语。
例句：I know a foreigner that is from Japan.
This is the pen (that) I wrote the letter with.
who，指代人，作主语、宾语。
例句：She is the girl who lives next door.
The people (who) we met in France have sent us a card.
whom，指代人，作宾语。
例句：I happened to meet the professor (whom) I got to know at a party in the shopping center yesterday.
whose，指代人、物，作定语。
例句：This is the house whose window broke last night.
as，指代人、物或事，作主语、宾语、表语。
例句：He is not the same man as he was. (as在从句中作表语)
易错提示：
（1）that只能引导限制性定语从句，不能引导非限制性定语从句。
（2）在限制性定语从句中，关系代词作宾语时可以省略，在非限制性定语从句中不可省略。
定语从句-关系副词
当关系词在定语从句中作状语时，要用关系副词。
when，指代时间，作状语。
例句：I still remember the day when I first came to Beijing.
where，指代地点，作状语。
例句：Can you tell me the office where he works?
why，指代原因，作状语。
例句：Do you know the reason why he was late?
限制性定语从句的特殊情况
(1) 关系代词作动词的间接宾语时，用to或for。
例：Who is the girl that he gave the flower to?
无关系代词时，也要用to或for。
例：The lady I wrote the poem for was my sister.
(2) 正式英语中，介词可置于从句之首，whom或which之前。
例：This is the great writer to whom our teacher was referring.
注意：介词不可置于who或that之前。
(3) 宾语从句的谓语如果是以介词结尾的短语动词，则不可把介词移至从句之首。
例：He received the email he was looking forward to.
(4) that和which的区别，参考that的“四用二不用”原则。
四用：
(1) 先行词为形容词最高级、序数词、all、much、little、few或被形容词最高级、序数词、all、much、little、few、the very、the only等修饰时。
例：This train is the first (one) that will go to Suzhou.
(2) 先行词为不定代词anything, everything, nothing时。
例：Do you have anything that you want to say for yourself?
(3) 先行词既有人又有物时。
例：Do you know the things and persons that they are talking about?
(4) 先行词是疑问代词which、who或主句以这些词开头时。
例：Who that has ever worked together with him doesn’t admire him?
二不用：
(1) 逗号后面不用that。
例：He passed the exam, which made his parents happy.
(2) 介词后面不用that。
例：This is the house in which Lu Xun once lived.
定语从句做题技巧
① 确定从句完整性；② 不完整选用关系代词；③ 从句完整，选用关系副词或介词+which/whom。