that引导名词性从句只起连接作用,在从句中不作任何成分,无意义,在宾语从句通常省略,但在主语从句、表语从句和同位语从句中均不能省略；what除引导从句外,还在从句中作成分,意为“所……的(东西)”,在含义上等于“名词+that”.
在从句练习中,我们经常看到that与what同时出现在选项中,许多同学因为区别不开,造成选错.下面我们谈谈如何解决这个难题.
一、引导词what与that在引导名词性从句的用法区别引导主语、宾语或表语从句时,what在从句中要充当句子成分；而that引导主语、宾语、表语和同位语从句时,不作任何成分,只起语法连接作用.试比较：
That George W. Bush won the presidency once again was what many people hadn't expected.乔治?布什再次赢得总统大选,这是很多人没有预料到的.
It is the same old story of not being grateful for what we have until we lose it.事情往往是这样,失去之物,方知珍惜.
二、引导词what与that在引导定语从句的用法区别首先,要记住在定语从句中,跟本不用what连接词.只有that和which.所以大家在做题中,如果是定语从句,就不要再考虑what了.That 在定语从句中,一般可以作从句的主语.宾语和表语.例如：
1、（2005北京春季卷） Do you still remember the chicken farm ____ we visited three months ago?
where B when C that D what
「分析」C 定语从句连词,that做从句visit的宾语.没有what连词.
2、He is not the naughty boy that he used to be.他不再是从前那个调皮的男孩了.
巩固性练习：从A、B、C、D四个选项中,选出可以填入空白处的最佳选项.
1. （2005福建卷） --Is that the small town you often refer to?
--Right, just the one _____ you know I used to work for years.
A that B which C where D what
2. ____ should be done must be done in time.
A What B All C Anything D All what
3. A computer is so useful a machine ______ we can use everywhere.
A that B which C as D what
4（2005天津卷）last month, part of Southeast Asia was struck by floods,from _____effects the people are still suffering.
A that B whose C those D what
5. （2005湖北卷） Her sister has becomes a lawyer, _____ she wanted to be.
A who B that C what D which
6. （07安徽） You can only be sure of _____ you have at present； you cannot be sure of something ____ you might get in the future.
A that； what B what；/ C which； that D /；that
7. （07陕西） _____ parents say and do has a life-long effect on their children.
A That B Which C What D as
8. （07上海） ____ he referred to in his article was unknown to the general readers.
A That B What C Whether D Where
9. （2006全国卷II）--What did your parents think about your decision?
----- They always let me do ____ I think I should.
A. when B that C.how D. what
10. （2005上海卷）The other day,my brother drove his car down the street at ___ I thought was a dangerous speed.
A. as B which C. what D. that
1. C 定语从句,one是先行词
2. A what引导主语从句,做从句中can的主语.这里what＝all that
3. C so……as……连用,属于定语从句的范围.
4. B 定语从句,whose从句effects的定语
5. D 非限定性定语从句,没有that 和what.
6. B 第一空是of的宾语从句,从句中have缺少宾语,因此用what做连词.第二空something后面是定语从句.
7. C 主语从句连词 "父母的所作所为对孩子有终身影响"
8. B主语从句连词,从句中refer to缺少宾语
9. D 是do的宾语从句
10. D 是介词at的宾语从句,at 做was 的主语.I think是插入语,可以忽略.
What 和that 都可以引导名词性从句.但他们在句子中的语法功能截然不同.
一．在引导名词性从句时,从属连词that一般要注意三个不（three nos）：
1．不省略（no ellipsis）
当从属连词that引导主语从句、表语从句和同位语从句时,that不省略.
Eg. 1) That he was able to come made us happy.(That 引导主语从句不可省略)
2)The truth is that I didn’t go there. (that引导表语从句不可省略)
3)The news that our team has won is true. (that引导同位语从句不可省略)
但在非正式文体中that 引导宾语从句时可以省略,除下列两种情况外：
that在宾语从句中不能省略的情况.
(1). 在主+谓+it(形式宾语)+宾补+that,从句(真正宾语) 的句型中不省略.
Eg: 4) He made it quite clear that he preferred to study English.（that不可以省略）
他很明确地说他宁愿学习英语.
(2). 由连词and连接的两个由that引导的宾语从句中,则第二个宾语从句前的that不省略.
Eg: 5) He told me that he would come and that he would come on time.（第一个That可以省略,但第二个that不可以省略.)他告诉我他将来而且他将准时来.
2．不作成分（no component）
that 在名词性从句中不作任何句子成分.
Eg. 6)Sydney kept his promise that he wouldalways do anything he could for Lucieto make sure of her happiness.（that在句中引导同位语从句,但在句中不作任何成分）
7))That you don’t like him is none of my business. (that在句中引导主语从句,但在句中不作任何成分,也不省略）
3． 没词义（no meaning）
that 在名词性从句中没有汉语意思.
Eg. 8)That she is a rich woman is known to us. (that在句中引导主语从句,但在句中没有汉语意义)
她是一个很有钱的女子,这一点我们都知道.
9）The thought that he might fail in the exam worried him. (that在句中引导同位语从句,但在句中没有汉语意义）
他也许会考试失败,这一想法使他很担心.
