定语从句中that和what
that可以引导定语从句.what只能用来引导名词性从句，一起来看看定语从句中that和what的用法。
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 would always do anything he could for Lucie to 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。
二.在引导名词性从句时，What一般要注意一个不(one no),一个有(meaning),一个作(component)：
1.不省略
what 引导名词性从句时，what 在句中不省略。这一用法与that 用法不尽相同。
eg. 10) What I had for breakfast today was rice with egg.(what 在句中引导主语从句，what 不省略)
11)----I think it’s going to be a big problem.
----Yes , it could be .
----I wonder what we can do about. (what在句中引导宾语从句，但在句中不省略)
2.有词义
what 引导名词性从句时，有词义。一般是“----的东西/事情/话等”这一用法与that 用法不一样
eg. 1) This is what she wanted to show us. (what在句中引导表语从句，在句中有词义，意思是“----的东西、事情)这是她想让我们看的东西。
12)What the teacher said had a good effect on the children. (what在句中引导主语从句，在句中有词义，意思是“----的东西、事情)老师所说的话对孩子们有很好的作用。
3.作成分
what 在引导名词性从句时，在从句中充当成分。这一用法与that 用法不一样。
Eg. 13)It doesn’t matter what he said.( it 是形式主语，what 在句中引导主语从句。What 在主语从句中作said 的`宾语。)
14)It was true ______Alice did surprised her mother.
A. that what B. what that C. that that D. all what
评析：这一题正确答案是A。it在句中是形式主语，that引导的是主语从句that Alice did surprised her mother.，在这个主语从句当中，what 又引导了主语从句what Alice did，what在从句中作did 的宾语。
三.要特别注意两点 ：
1. What所引导的从句可以在介词后作宾语，而that从句一般不可以。
Eg： 15)After months of voyage ,Columbus arrived in _____later proved to be a new continent.
A. where B. which C. what D. that
评析：答案是C。因为在介词in 后面，应该用what引导的宾语从句，what在宾语从句中作主语。而that不可以这样用。
16) On stepping into his room, he was astonished to find the floor covered with _____looked like tiny insects.
A. that B. something C. what D. anything
评析：答案是C。因为在介词with 后面，应该用what引导的宾语从句，what在宾语从句中作主语。而that不可以这样用。
2. that可以引导定语从句，而且是定语从句引导词中的顶梁柱，它在定语从句中要充当成分， 通常作定语从句中的主语、宾语。同时that也可以引导名词性从句，在名词性从句中that 不充当成分。 但What不可以引导定语从句。它只可以引导名词性从句,在句中相当于 what =all that ,what 在名词性从句中充当成分，常常作主语、宾语和表语。
Eg: 17).All _____I want to know is what he meant .
A. that B. which C. what D. whatever
评析：答案是A.本题是that引导的定语从句。而what不可以引导定语从句。What=all that .又因为先行词是all ，故其后不可以接which 引导的定语从句。而. Whatever=anything that /no matter what 从意义和语法都不符合。所以只能选A,that.
18) ______impressed me most was ______such a little boy could play the violin so well.
A. That ; what B. What ; that C. That ; that D. What ; what
评析：答案是B.第一空是what 引导的主语从句，并在主语从句中作主语。第二空是that 引导的表语从句，that在表语从句中不省略，不作成分，也没词义。
