定语从句what的用法
what是不能引导定语从句的.that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.
1）what = the thing which；whatever = anything
What you want has been sent here.
Whatever you want makes no difference to me.
2) who= the person that whoever= anyone who
（错）Who breaks the law will be punished.
（错）Whoever robbed the bank is not clear.
（对）Whoever breaks the law will be punished.
（对）Who robbed the bank is not clear.
3) that 和 what
当that引导定语从句时 ，通常用作关系代词，而引导名词性从句时，是个不充当任何成分的连接词。宾语从句和表语从句中的that常可省略。What只能引导名词性从 句， 用作连接代词，作从句的具体成分，且不能省略。
I think (that) you will like the stamps.
What we need is more practice.
