用来引导定语从句。它代表先行词，同时在从句中作一定的句子成分。
关系代词有主格，宾格和属格之分，并有指人与指物之分，它在先行词和定语之间起纽带作用。
主要的关系代词有who,whom,whose,which和that。
①who指人，在定语从句中作主语或宾语；
whom指人，在定语从句中作宾语；
whose在定语从句中作定语，其先行词既可以是人也可以是物。
例如：The teacher who/whom you want to see has come.
②which指物，在定语从句中作主语或宾语。
例如：Here is the coat which will be made to you.
③that多指物，有时也指人，在定语从句中作主语或宾语。
例如：I have eaten bread that/which he gave to me.
关系代词的基本用法：
（1）引导定语从句
关系代词代表先行词，同时在从句中作一定的句子成分，例如：
The girl whom I spoke to is my cousin. 跟我讲话的姑娘是我表妹。
（该句中whom既代表先行词the girl，又在从句中作介词to的宾语。）
This is the pencil whose point is broken.这就是那个折了尖的铅笔。
(whose 指物，在限定性定语从句中作定语)
He came back for the book which he had forgotten.他重新回来都忘记了他的那本书。
（which指物，在限定性定语从句中作宾语，可以省略）
（2）which为其先行词。例如：
He said he saw me there, which was a lie.他说在那儿看到了我，纯属谎言。
（3）其他用法
关系代词that在从句中作宾语或表语时可省略， 例如：
I've forgotten much of the Latin I once knew.我过去懂拉丁语，现在大都忘了。
He's changed. He's not the man he was.他变化很大，已不是过去的他了。
注释：关系代词在从句中作宾语时可省略，that在从句中作表语时也可省略。
非限定性定语从句中，不能用that作关系代词。
关系代词的语法注意:
1.常用that 不用which的情况:
①先行词为all, anything, everything, nothing, something等时。
I will do all (that) I can to help you.
②先行词被序数词、形容词最高级或the only, the very等所修饰时。
He was the only person that joined the army in his village that year.
③先行词既有人,又有物时
They talked about the people and the things (that)they remembered in the school.
④当主句是以who 或which开头的疑问句时, 定语从句用that 引导, 以避免重复。
Who is the boy that was here just now?
2. 若被修饰的名词充当从句介词宾语时
介词可提前于引导词之前,此时的引导词只能用whom 或which.
The pencil with which he was writing broke.
3. 当先行词为way时
其后的定语从句用in which或 that引导,也可不用引导词。
I don’t like the way (in which/that) he talks to me.
used to/be used to的分别：
①used to + do："过去常常"表示过去习惯性的动作或状态，但如今已不存在。
Mother used not to be so forgetful.
②be used to + doing： 对……已感到习惯，或"习惯于"，to是介词，后需加名词或动名词。
Scarf is used to taking a walk.(现在习惯于散步)
③used to 的用法 （否定式简写为usedn't） 过去经常，以前常常
This used to be a shabby house. （此房年久失修）
used to,would这两个词语都可以表示过去常做某事，有时可以换用。
used to do 强调过去习惯性的行为或状态，但是现在没有这种行为或状态了。因此，这个短语的内涵是今昔对比。
关系代词的用法:
一、不用that的情况
在引导非限定性定语从句时 (错)The tree, that is four hundred years old, is very famous here.
介词后不能用We depend on the land from which we get our food.
二、只能用that作为定语从句的关系代词的情况
a.在there be 句型中，只用that，不用which。
b.在不定代词，如：anything, nothing, the one, all, much, few, any, little等作先行词时，只用that，不用which。
c.先行词有the only, the very.（恰恰，正好）.any. few. little. no. all 修饰时，只用that。
d.先行词为序数词、数词、形容词最高级时，只用that。
e.先行词既有人，又有物时。
Finally, the thief handed everything that he had stolen to the police.那贼最终把偷的全部东西交给了警察。
三、不能用关系代词Which 的几种情况
1.当先行词为all, little, much, few, everything, none 等不定代词时关系代词用that而不用which.
There are few books that you can read in this book store.
2.当先行词既有人又有物时，关系代词用that, 不用which.
He asked about the factories and workers that he had visited.
3.当先行词有形容词最高级修饰时，关系代词用that, 而不用which.
Guilin is one of the most beautiful cities that I have ever visited.
4.当先行词有序数词修饰时，关系代词用that, 不用which.
The first sight that was caught at the Great Wall has made a lasting impression on him.
5.当先行词被the only, the very, the same, the right等修饰时，关系代词用that,不用which.
It is the only book that he bought himself.
6.当先行词被all, every, any, much, little, few, no等修饰时，关系代词用that,不用which.
There is no clothes that fit you here.
7.当主句是以which开头的特殊问句时，定语从句关系代词一般用that而不用which.
Which of the books that had pictures was worth reading?
8.在强调句型" It is ... that ..." 中,只用that,不用which.
It is in this room that he was born twenty years ago.
9.在" such (the same) ... as ..." 句型中,关系代词要用as, 而不用which.
We need such materials as (not which) can bear high temperature.
10.表示" 正如... 那样 "，" 正象..." 之意时，用as, 不用which.
Mary was late again, as had been expected.
11.As 引导的定语从句可以放在主句前面，也可以放在主句后面或主句中间，而由 which引导的定语从句只能放在主句后面。
As we all know, oceans cover more than 70% of the earth.
关系代词可以省略的情况:
1.关系代词which,whom,who,that在定语从句中作宾语时可以省略。
Here is the man (who/that/whom)you're been looking for.
2.当that在从句中作补语时可以省略。
She is all (that) a teacher should be.
3.在there be 结构中出现定语从句，或在定语从句中出现there be 时，用作主语的关系代词也可省略。
This is the best dictionary (that) there is in the library.
4.状语的省略
（1）当先行词是reason，而且定语从句中作原因状语时，关系代词可用why,that,也可以省略。
That is the reason (why) I did it.
（2）当先行词是way,且在定语从句中作方式状语时，关系代词可用in which,that,也可以省略。
I don't know the way that/ which leads to the top of the mountains.
（3）当先行词是time时，关系代词可用when,that 或省略。
I don't know the exact time (when/that) the sports meet will take place.
注意：关系代词作宾语有时也不能省略。
关系代词不能省略情况：
1.在介词+whom与介词+which中，whom,which不能省略。
That is the headmaster with whom my father was talking just now.
2.在非限制性定语从句中，作宾语的关系代词也不能省略。
The elephant is like a spear,as anybody can see.
3.在the same ...as,such...as,as...as,the same...that中，as,that即使作宾语，也不能省略I have bought the same bicycle as you have (bought). (指同类用as)
This is the same book that I read the day before yesterday. (that指同一个)
4.当由and,but,or等连词连接两个或两个以上的定语从句修饰一个先行词时，第一个关系代词可省，但第二，第三个不可省略。
This is the book (which) I read yesterday and which I find very interesting.
关系代词格式：
关系代词有主格，宾格和属格之分，并有指人与指物之分。that的用法最广，that可指人也可指物。
见表：
限定性 | 非限定性 | 限定性 | |
主格 | who | which | that |
宾格 | whom | that | that |
属格 | whose | of which | of which |
特殊关系代词的用法：
关系词通常是用来引导定语从句的,但as,than和but这三个词却与which, who, that, where, when和why等典型的关系词不同。
三者本身具有常见的字面意义,前面有名词作先行词,但后面的定语从句却不像典型的定语从句。
这些既像连接词又像关系词的词,我们就姑且称它们为准关系词。
一、as作为准关系代词出现在the same ... as, such ... as, 和as ... as等结构中。
这时, as前面通常要有名词;
as后面的从句意义上不像是通常的定语从句;
as需要在从句中充当一定的句子成分。
1. the same ... as
1) as在从句中作主语
She is the same person as came here last Sunday. 她就是上星期天来这儿的那个人。
2) as在从句中作宾语
This is the same watch as I lost while I was doing the sightseeing in Shanghai three months ago. 这块表跟我三个月前在上海观光时丢失的那块一样。
3) as在从句中作be动词的表语
China is not the same country as it used to be. 中国已今非昔比。
4) as在从句中作行为方式状语
You should learn to do housework in the same way as your mother does it. 你应该学会像你母亲那样做家务活。
2. such ... as
1) as在从句中作主语
Let children read such books as tend to make them better and brighter. 让孩子看那些容易让他们变得更好更聪明的书。
2) as在从句中作宾语
This is not such an interesting book as you just talked about. 这不是你们刚才谈到的那种有趣的书。
3) as在从句中作be动词的表语
Such a bad man as he is will be punished. 像他这样的坏人一定会受到惩罚。
3. as ... as
1) 第二个as在从句中作主语
He has as much money as is required. 他要多少钱就有多少钱。
2) 第二个as在从句中作宾语
I have as many friends as you (do). 我的朋友和你的朋友一样多。
3) 第二个as在从句中作be动词的表语
As is known to all, Taiwan is one of the Chinese provinces。众所周知,台湾是中国的一个省。
二、than作为准关系词 出现在more ... than, less ... than, fewer ... than等结构中。这时, than前面要有名词; than本身有比较意义,而且需要在从句中充当一定的句子成分。
1. than在从句中作主语
Take it easy, I have more money than is needed. 放心，我的钱比所需要的还要多。
2. than在从句中作宾语
I have more/fewer books than you (do). 我的书比你的多/少。
3. than在从句中作be动词的表语
He is a better boy than you (are). 他是一个比你更好的男孩。
三、but作为准关系词 本身含有否定意义,它前面的主句还有另一个否定意义。这时but在意义上相当于“who ... not ...”或“which/that ... not ...”。
1.定语从句中有be动词,并将其改为含but的复合句时, be动词后面不能再有not
There is no one who is not conccrned about his future. (= There is no one but is concerned about his future.) 没有人不关心自己的未来?
2. 定语从句中有can, will, shall, must, may, should, ought to等助动词或情态动词,在将其改为含but的复合句时,这些助动词或情态动词后面不能再有not
There is nothing that she can’t do. (= There is nothing but she can do.) 没有她做不到的事?
3. 定语从句中有助动词do, does, did, 在将其改为含but的复合句时,应去掉do, does, did, 还原其后动词的时态及人称变化
There is no one who doesn’t wish to make great achievements. (= There is no one but wishes to make great achievements.) 谁都希望自己将来能有所成就。
四、which的先行词：关系代词which的先行词可以是一个句子
He said he saw me there, which was a lie。他说在那儿看到了我，纯属谎言。
说明：关系代词that在从句中作宾语或表语时可省略
I've forgotten much of the Latin I once knew。 我过去懂拉丁语，现在大都忘了。
相关初中英语知识点：定语从句
定语从句：
担任定语动能的句子称为定语从句。定语从句在句中的作用相当于形容词，故又称为形容词性从句。
定语从句所修饰的词叫先行词；定语从句一般用关系代词或关系副词来引导，关系词放在先行词与定语之间起连接作用，同时又作从句中的一部分。
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：
关系词 | 词形 | 所修饰的先行词 | 在从句中所作的成分 |
关系代词 | who | 人 | 主语、宾语、表语 |
whom | 人 | 宾语 | |
which | 物 | 主语、宾语、表语 | |
that | 人或物 | 主语、宾语、表语 | |
as | 人或物 | 主语、宾语、表语 | |
whose=of whom/of=which | 人或物 | 定语 | |
关系副词 | when | 时间词 | 时间状语 |
where | 地点词 | 地点状语 | |
why | 原因词 | 原因状语 |
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 party
in 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。有时也可和一些介词一起引导定语从句。
例如：Do you remember the time when she went abroad?
2. where的用法：它的先行词通常有place, spot, room等。
例如：I like places where the weather is always warm.
3. why的用法：它的先行词只有reason。Why时常也可以省略。
例如：Tell me the reason why she was late.
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