定语从句的分类：根据定语从句与先行词的关系，定语从句可分为限制性定语从句及非限制性定语从句。
限制性定语从句紧跟先行词，主句与从句不用逗号分开，从句不可省去;非限制性定语从句主句与从句之间有逗号分开，起补充说明作用，如省去，意思仍完整。
1．由that引导的定语从句一般情况下that可指人或物，可以代替who，whom和which，在从句中作主语或谓语动词的宾语，但不能作介词的宾语。
如：The comrade that（who）is speaking at the meeting is an advanced worker．正在会上讲话的那个同志是个先进工人。
Is this the doctor that（whom）you talked about yesterday？这就是你们昨天谈论的那位医生吗？The letter that（which）I received yesterday was from my father．昨天我收到的信是我父亲寄来的。
2．由who，whom和whose引导的定语从句who在从句中作主语，whom在从句中作宾语，whose在从句中作定语。
如：This is the thief who stole my bike．这就是偷我自行车的那个贼。
He is the boy whom you wanted to find．他就是你想要找的那个男孩。
The girl whose mother is a doctor is waiting for you outside．妈妈是个医生的那个女孩在外面等你。
3．由which引导的定语从句which指物，在从句中作主语或谓语动词的宾语。
如：The book which Mum bought me last week is called“Red Star Over China”．妈妈上周买给我的那本书名叫《西行漫记》。
The house which Lu Xun once lived in is now the Lu Xun Museum．鲁迅曾经住过的那所房子现在是鲁迅博物馆。
4．由where，when和why引导的定语从句where在从句中作地点状语，when在从句中作时间状语，why在从句中作原因状语。
如：I will never forget the day when I joined the League．我永远忘不了我入团的那一天。
He will go back to the school where he studied next week．下周他要回到他曾经学习过的学校。
I don't know the reason why he quarreled with Zhang Lin．我不知道他同张琳吵架的原因。
［注意点］关系代词在定语从句中作宾语时，关系代词常省去。
如：The play（which）I watched just now had been on twice on Channel我刚才看的那个戏剧在3频道上已上演了两次。
限定性定语从句：1. that即可代表事物也可代表人，which代表事物；它们在从句中作主语或宾语，that在从句中作宾语时常可省略关系词，which在从句中作宾语则不能省略。
而且，如果which在从句中作“不及物动词＋介词”的介词的宾语，注意介词不要丢掉，而且介词总是放在关系代词which的前边，但有的则放在它原来的位置2. which作宾语时，根据先行词与定语从句之间的语义关系，先行词与which之间的介词不能丢3. 代表物时多用which，但在带有下列词的句子中用that而不用which，这些词包括all, anything, much等，这时的that常被省略4. who和whom引导的从句用来修饰人，分别作从句中的主语和宾语，whom作宾语时，要注意它可以作动词的宾语也可以作介词的宾语5. where是关系副词，用来表示地点的定语从句6. when引导定语从句表示时间〔注〕值得一提的是，表示时间“time"一词的定语从句只用when引导，有时不用任何关系代词，当然也不用that引导By the time you arrive in London, we will have stayed there for two weeks.I still remember the first time I met her.Each time he goes to besiness trip, he brings a lot of living necessities, such as towers, soap, toothbrush etc.7. whose是关系代词，修饰名词作定语，相当于所修饰成分的前置所有格8. 当从句的逻辑主语是some, any, no, somebody, anybody, nobody, something, anything, everything或nothing时，常用there is来引导非限定性定语从句非限定性定语从句的作用是对所修饰的成分作进一步说明，通常和主句间用逗号隔开，将从句拿掉后其他部分仍可成立1. which引导的非限定性定语从句来说明前面整个句子的情况或主句的某一部分2. 在引导限定性定语从句时，that有时相当于in which, at which, for which或at which Attitudes towards daydreaming are changing in much the same way that(in which)attitudes towards night dreaming have changed.人们对白日做梦的态度正在改变，这与人们对夜间做梦的看法的变化有非常相似之处。
I like the music for the very reason that(for which) he dislike it.我出于某种原因喜欢这种音乐，而他恰恰与我相反。
We arrived the day that(on which) they left.刚好我们到的那天他们走了。
3. 有时as也可用作关系代词4. 在非限定性定语从句中，不能用that，而用who, whom代表人，用which代表事物 限定性定语从句口诀:（口诀一）定语从句真奇妙，关系代(副)词来引导，定语从句分两种，是否限定看逗号。
（口诀二）which指物who指人，地点where时间when,that人或物均可，why之前是reason。
（口诀三）从句当中作宾语，whom可以代替who,为了句子更简练，关系代词可省去。
（口诀四）关系若是表所属，whose用法不可无。
除此之外还有啥，whom, which加of。
（口诀五）关系代词不一般，介词经常用在前，此时不用that, who，which, whom才安全。
引导定语从句的关系词：关系代词的用法：1. 关系代词的句法功能(1)关系代词在句中作主语例如：I prefer music that/which has great lyrics(2) 关系代词在句中作宾语例如：I like music that I can sing along with.(3)关系代词在句中作表语例如：The house is not the one (that) it used to be.2. 关系代词在从句中作名词的定语例如：Is she the teacher whose hair is very short?3. 关系代词的用法(1)who和whom的用法：二者都用于指人。
Who在定语从句中作主语、宾语，whom 在定语从句中作宾语。
Whom在从句中能坐介词的宾语，而who则不能。
例如：I happened to meet the professor (who/whom) I got to know at a partyin the shopping center yesterday.(2)whose的用法：一般指人，有时也指物。
在定语从句中作定语。
例如：I have got a friend whose brother is training for the Olymoics.(3)which的用法：一般指物，在定语从句中作主语、宾语、偶尔作定语。
例如：The photo which we liked best was taken by Zhao Min.(4)that的用法：指人或物，指人时可与who、whom互换，指物时可与that互换。
在从句中可作主语、宾语，还可作表语。
例如：It is a book (that/which) no one really likes.4. 宜用who不宜用that的情况(1)先行词是指人的不定代词时例如：Congratulations to our winners and thanks to everyone who entered the competitions.(2) 先行词指人时且含有较长的后置定语例如：The boy was crying hard who lost his way while looking for his mother. 5. 在以下情况中，关系代词常用that(1)先行词为all, little, few, none 及some-, any-, no-与thing所组成的复合单词时，只能用that。
例如：There is nothing that will stop us making progress.(2)先行词前有only, some, any等形容词修饰时，只能用that。
例如：This is the only problem that we can’t work out.(3)先行词前由形容词的最高级或序数词修饰时，只能用that。
例如：This is the most interesting book that I have ever read.(4)先行词同时包括人和物时，只能用他和她，例如：Lu Xun and his works that we are familiar with are considered great.6. 只能用which不能用that的情形(1)当非限制性定语从句中的先行词指物时，只能用which例如：The pencil-case, which I bought last week, is missing.(2)关系代词前面有介词时，只能用which例如：The tree under which we used to take a rest has been cut down.(3)先行词本身就是that时例如：I don’t like that which he did.关系副词的用法1. when的用法：它的先行词通常是time, day, morning等。