定语从句中能不能用what?如果可以，那么大家知道怎么用？
what是不能引导定语从句的可以引导定语从句.
what只能用来引导名词性从句,比如主语从句,如：What he said was not ture.（他说的不是真的.） 或者宾语从句,如：I don't believe what he said.（我不相信他说的话.）而定语从句属于形容词从句.
（that可以引导的从句类型较多,只回答在定语从句中的用法.）
that作为关系代词,引导定语从句,修饰其先行词.如：All that glitters is not gold.（发光的不都是金子）定语从句that glitters修饰先行词all,起形容词作用,所以定语从句属于形容词从句.在这句话中,that其实是代替了先行词all,在从句中作glitter的主语.
that也可以在从句中作宾语,有时可以省略掉,如：The car (that) I hired broke down after five kilometers.(我租来的车五公里后就抛锚了.）that替代car,在从句that I hired中作hire的宾语.
你可以比较以下一组句子.
What I said is true.我说的是实话.（what引导名词从句,在句中作主语,what前没有任何东西）
All that I said is true.我说的一切是实话.（that引导定语从句,在句中作定语,all是先行词,that I said修饰all）
定语从句中能不能用what?
定语从句（Attributive Clauses）在句中做定语,修饰一个名词或代词,被修饰的名词,词组或代词即先行词.定语从句通常出现在先行词之后,由关系词（关系代词或关系副词）引出.
关系代词有：who,whom,whose,that,which等.
关系副词有：when,where,why等.
关系代词引导的定语从句
关系代词所代替的先行词是人或物的名词或代词,并在句中充当主语、宾语、定语等成分.关系代词在定语从句中作主语时,从句谓语动词的人称和数要和先行词保持一致.
1）who,whom,that
这些词代替的先行词是人的名词或代词,在从句中所起作用如下：
Is he the man who/that wants to see you?
他就是你想见的人吗?（who/that在从句中作主语）
He is the man whom/ that I saw yesterday.
他就是我昨天见的那个人.（whom/that在从句中作宾语）
2) Whose 用来指人或物,(只用作定语,若指物,它还可以同of which互换）,例如：
They rushed over to help the man whose car had broken down.那人车坏了,大家都跑过去帮忙.
Please pass me the book whose (of which) cover is green.请递给我那本绿皮的书.
3）which,that
它们所代替的'先行词是事物的名词或代词,在从句中可作主语、宾语等,例如：
A prosperity which / that had never been seen before appears in the countryside.农村出现了前所未有的繁荣.（which / that在句中作宾语）
The package (which / that) you are carrying is about to come unwrapped.你拿的包快散了.(which/that在句中作宾语）
关系副词引导的定语从句
关系副词可代替的先行词是时间、地点或理由的名词,在从句中作状语.
1）when,where,why
关系副词when,where,why的含义相当于"介词+ which"结构,因此常常和"介词+ which"结构交替使用,例如：
There are occasions when (on which) one must yield.任何人都有不得不屈服的时候.
Beijing is the place where (in which) I was born.北京是我的出生地.
Is this the reason why (for which) he refused our offer?这就是他拒绝我们帮助他的理由吗?
2）that代替关系副词
that可以用于表示时间、地点、方式、理由的名词后取代when,where,why和"介词+ which"引导的定语从句,在口语中that常被省略,例如：
His father died the year (that / when / in which) he was born.他父亲在他出生那年逝世了.
He is unlikely to find the place (that / where / in which) he lived forty years ago.他不大可能找到他四十年前居住过的地方.
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