定语从句：在复合句中作定语用修饰句子中某一名词或代词的从句叫定语从句.
① 定语从句的位置：放在名词或代词的后面.如：The man who has an umbrella in his hand is my uncle. (手上抓了一把雨伞的人是我的叔叔.)
② 语法术语的改变：被修饰的名词或代词称为先行词；引导定语从句的连接词称为关系词,其中that、which、who称为关系代词,where、when、how称为关系副词.
③ 关系代词或关系副词的作用：
关系代词who、whom 和whose指人,分别在从句中作主语、宾语和定语.which指物that多指物, 有时也指人,它们在从句中可以作主语或宾语.定语从句中关系代词作从句宾语时可以省略.This is the thief (that/who/whom) we have been looking for these days. (这个就是我们近日一直在找的小偷.) / Please find a room which is big enough for all of us to live in. (请找一间足够大能住下我们全体的房间.)
关系副词when或where引导定语从句时,它们在从句中分别作时间状语和地点状语.如：This is the room where they had a quarrel a week ago. (这是一个星期前他们吵架的房间.) / I can never forget the day when I first saw you. (我永远不会忘记第一次见到你的日子.)
④ 限制性定语从句和非限制性定语从句.限制性定语从句在句中不能省略,否则主句意思就不完整.非限制性定语从句和主句之间往往用逗号分开,这种从句是一种附加说明,如果从句子中省去,也不致于影响主句的意思.如：She spent the whole evening talking about her latest book, which none of us had heard of. (她一个晚上都在谈论她最近的书,我们一个人都没有听说过那本书.)
[注解]
1、关系代词只能用that的情况：当先行词是指事物的不定代词(all, anything等),或先行词部分含有最高级,或含有序数词时,不能用其他的关系代词,只能用that. 如：All that Lily told me seems untrue. (Lily告诉我的话似乎不真实.) / Can you give me anything that has no sugar inside? (能不能给我点里面没有糖的东西?) / This is the first two-story bus that runs in our city. (这是第一辆运行于我市的双层公交车.)
2、关系代词在从句中作主语时一般用who(指人)、that(指物),而不用that指人或用which指物. 如：Do you know the woman who is weeping in the corner? (你认识那个在角落里哭泣的女人吗?) / Do not tell me any words that have nothing to do with me. (跟我无关的话,请一个字也不要说.)
3、关系代词作从句的宾语时,常用which或whom,较少使用that或who,而且,关系代词常常省略.如：Tom’s father was the first parent whom our teacher talked to. (Tom的父亲是我们老师第一个谈话的人.)
4、当关系代词紧跟在介词后面时,必须用which或whom,而当介词移到句尾时,则又可以使用that或who. 如：This is the room in which Miss Li once lived. (= This is the room Miss Li once lived in.) (这是李小姐曾经居住过的房间.)
(5) 主语从句：在句子中充当句子主语的从句叫主语从句.位于谓语动词之前.通常由that,whether以及疑问连词引导.一般情况下,常用it替代主语从句,而将主语从句移到句尾.如：When we should start is still a question. (我们该在什么时候开始还是个问题呢.)