1、that的用法
主要特点有：
①that在定语从句中作主语时不省略,作宾语时可以省略；
②that不引导非限制性定语从句；
③that前不加介词
例子Theforeigners that visited our school yesterday are from Canada.
昨天参观我们学校的外国人来自加拿大。(that代替人作主语)
只能用that引导定语从句的情况:
①先行词被序数词或形容词最高级修饰；
②先行词是不定代词all, the one, everything等；
③先行词被 any , the only , the last等修饰；
④先行词既指人又指物时。
例子Alice is the only person ( that ) I can trustin the office.
爱丽丝是办公室里我唯一信任的人。(先行词被the only修饰只用that)
2、which的用法
先行词是物,在定语从句中作主主语或宾语。
主要特点：
①which在定语从句中作主语时不能省略,作宾语时可以省略,引导非限制性定语从句时不能省略；
②which的前面可以有介词;
③which的先行词可以是前面数个的的句子,意为”这使…….;这一点……”。
例子 A computeris a machine which can do many things for us.
电脑是一种可以为我们做许多事情的机器。( which作主语)
3、who的用法
who的先行词是人。who在定语从句中作主语、宾语或表语。
例子 I like theboy who gets along well with his classmates
我喜欢那个与他的同学相处得好的男孩。
先行词为one、ones、anyone或 those时,或双重定语从句中一个定语从句的关系代词是that，另一个指人的关系代词词用who。
扩展资料
在定语从句中which和that用法区别：在定语从句中，which 和that 在指代事物时，一般可以互换使用，但并非在任何情况下都是这样，几种宜用that,而不宜用which 的情况如下：
1、先行词为不定代词,all,much,something,everything,anything,nothing,none,the one等。
2、先行词被only, any, few, little, no, just, very, one of等词修饰时。
3、先行词是序数词时或被序数词修饰时。
4、先行词是最高级或被最高级修饰时。
5、先行词既有人又有物,用which和who都不适合,这时宜用that。
6、被修饰词为数词时。
7、如果有两个从句,其中一个关系代词已用which ,另一个关系代词宜用that,以避免语言的单调或重复。
8、疑问词是who或which,关系代词宜用that,以避免重复。
9、主句是There be 结构,修饰其主句的定语从句宜用that 作关系代词。
10、被修饰成分为表语时,或者关系代词本身是定语从句的表语时,该关系代词宜用that。
定语从句中宜用which而不宜用that 的情况：
1、当关系代词的前面有介词时。
2、在非限制性定语从句中。
3、在一个句子中有两个定语从句,其中一个定语从句的关系代词用了that, 另一个宜用which。
4、当关系代词后面带有插入语时。
5、先行词本身是that, 宜用which。
6、先行词是those+复数名词。
主要看修饰的对象。that既可指人也可指物；who只能指人；which只能指物。
He is the man who/ that I saw yesterday.（who/that在从句中作宾语）
A friend who/ that helps you in time of need is a real friend . （who/that在从句中作主语）
一.只能用that作为定语从句的关系代词的情况 ：
1) 在不定代词,如：anything, nothing, everything, all, much, few, any, little等作先行词时,只用that.
All that is needed is a supply of oil.
2) 先行词有the only, the very, the just修饰时,只用that.
He is the very man that helped the girl out of the water .
3) 先行词为序数词(the last)、数词、形容词最高级时,只用that..
The first English book that I read was “The Prince and the Pauper” by Mark Twin .
4) 先行词既有人,又有物时.
He talked about the teachers and schools that he visited .
5)当主句是以who 或which 开始的特殊疑问句时,用that 以避免重复.
Who is the person that is standing at the gate .
二.不能用that的情况，而用which/who：
非限定性定语从句，逗号后面只能用which。
The sun heats the earth, which is very important to us.
关系代词前有介词时.
This is the hotel in which you will stay.
如有两个定语从句,其中一句的关系代词是that,另一句宜于用which.
Let me show you the novel that I borrowed from the liberary which was newly open to us.
who,that在很多情况下可以通用,但是有些情况只能用who:
1.先行词是one,ones,anyone的时候宜用who.
Anyone who fails to finish the task given should be criticized.
2.先行词为those的时候,宜用who为关系代词
Those who want to go to the Great Wall sign up here.
3.当先行词有比较长的后置定语的时候
I met a foreigner in the street yesterday who could asked me questions in
Chinese.
4.一个句子中带有两个定语从句,其中一个定语从句的关系代词为that,另一个则用
who.
The boy that you met last night is the group leader who studies very hard.
5.在there be 开头的句子中
There is an old man who wants to see you.
that常用作关系代词，可指代某人，也可指代某物，指物时有的时候可用which替换that，指人时可用who替换。但在下列情况下，一般用that。
一、that指代某物事时
1.先行词为all, few, little, much, the one, something, anything, everything, nothing等时。如：
We'll do all that we can to protect the endangered plants and animals.
2. 先行词被all, every, no, some, any, (a)few, (a)little, much等限定词修饰时。如：
You can borrow any book that you want to read in our school library
3. 先行词被序数词或形容词最高级修饰时。如：
This is the most beautiful city that I've ever seen.
4. 先行词被the only, the very, the right, the last 等修饰时。如：
This is the very factory that they visited last summer holiday.
5. 先行词是并列结构，既有人又有物时。如：
He talked happily of the writer and his books that interested him.
6. 先行词前有the same 修饰，表示和先行词是同一物时。如：
This is the same purse that I lost yesterday
7. 先行词为数词时。
Look at the books on the bookshelf. You can see the two that you bought me for my birthday.
8. 如果which引导的定语从句中又含有一个定语从句，为避免重复，第二个定语从句用that。如：
They built a factory which produced things that had never been seen.
9. 以which作主语开头的特殊问句，为了避免重复，定语从句用that。如。
Which is the bus that you will take?
10. 关系代词在定语从句中作表语时，而且通常省略。如：
My hometown is no longer the place that it used to be.
11. 关系代词在there be 句型中作主语时,而且常可以省略。如：
This is the fastest train (that) there has ever been.
二、that 指代某人时。
1. 泛指某人时。如：
He is a man that is never at a loss.
2. 主句是以作主语的who开头的特殊问句，为了避免重复时。如：
Who is the person that is talking with our headmaster?
3. 先行词前有the same时。如：
This is the same man that gave us a talk last year.
4. 关系代词在定语从句中作表语时。如：
He was not the man (that) he was 10 years ago.
另外，that也可用作关系副词，表示时间、地点、原因和方式，在口语中可以用来代替when, where, why 引导的定语从句。在以下名词day, year, time, moment, reason, place等作先行词时，可用that作关系副词引导定语从句。如：
I'll never forget the day (that) I joined the League.
This is the first time that I have been abroad for traveling.
（注：先行词是time,前面有序数词或last修饰时，常用that引导定语从句或者省略。）
当先行词为表示方式的词the way时，可用that引导定语从句, that常可以省略。
I don't the way you speak to her.
先行词为everything，all，little，much等不定代词时；
先行词被all，every，no，any，some，little，much，one修饰时；
先行词被the only，the very，the same，the last修饰时；
先行词被序数词或形容词最高级修饰时；
先行词是be的表语或there be 的主语时；
先行词有人又有物时；
当主句是以who或which开头的特殊疑问句时。
不能用that的情况
介词前置时；
非限定性定语从句中
先行词本身是that
语法说难不难说易不易，多做做题目，积累下，就好了本回答被提问者采纳