许多同学在学习复合句和特殊句式时，都会为“that”和“what”的选择而头疼， 感觉这两个连接性词语辨别起来“有点乱”。你也不妨先做一做下面几个题，看看你能否把他们分清：
1.A modern city has been set up in ____ was a wasteland ten years ago.
A. what B. which C. that D. where
2.Because of my poor memory, all ____ you told me has been forgotten.
A. that B. which C. what D. as
3.With his work completed, the businessman stepped back to his seat, feeling pleased ____ he was a man of action.
A. which B. that C. what D. whether
4.It is what you do rather than what you say _______ matters.
A. that B. what C. which D. this
5.His plan was such a good one _________ we all agreed to accept it.
A. so B. and C. that D. as
上面5个题中的前四个题，在选项中都同时出现了what和that，如何选择呢？
第1题为答案A。考查名词从句连接代词what的运用。“十年前是一片荒地的地方建起了一座现代化的城市”。作介词in的宾语不能用that，因为名词从句中that不做任何成分；连接代词what引导一个宾语从句，“是一个什么样的地方”；
第2题为答案A。考查定词从句关系代词that的运用，“因为记忆不好，你告诉我的，我都给忘了”。句中先行词为“all”，that为关系代词，意指all，在从句中做宾语；
第3题答案为B。考查名词从句从属连词that的运用。“…他很得意自己是个干事的人。”陈述了一个事实，故选择从属连词that
第4题为答案A。考查强调句结构中的that。“It is (was) 所强调的成分 that (who) 其他成分。”
第5题为答案C。考查结果状语从句中的从属连词that。“他的计划非常好，我们都同意接受。”
从上面的分析中，我们不难看出，“that”一词非常活跃，在定语从句、名词从句、状语从句和强调句中都有使用；而命题设项时，“what”又常常用来作为干扰，因此，要想辨清“that” 和“what”这两个词，突破复合句式的这个难点，就要注意：
一、定语从句的关系代词中没有what，因此要牢记what不会引导定语从句。
例如：
1.The thought of going back home was kept him happy while he was working abroad.
A.that B.all that C.all what D.which
答案为B。这里的all是先行词，而that是关系代词，在从句中做主语。许多同学将此题误选成了C。需要注意的是，C选项前面要是去掉all，则变成了what引导的名词从句，则就是对的了。
2.You can only be sure of_____ you have at present; you cannot be sure of something ___ you might get in the future. [2007 安徽卷]
A. that; what B. what; /
C. which; that D. /; that
答案为B。前一个空为名词从句连接代词what，(从句做of的宾语，what又为get的宾语)，后一个空为定语从句(先行词为something，关系代词做get的宾语，关系代词省略)。
二、名词从句中同时存在that和what这两个连接性词语，可以从其语法地位和意思上来辨别。
that 引导名词性从句时，没有任何含义和语法作用，只是提示引导了名词从句；而what引导名词性从句时，在主从句中都要充当一定的句子成分，what的意思是“什么…”
例如：
1.The mountain is no longer ____ it used to be.
A. which B. that C. what D. as
答案C 。“山不是以前那个样子了”。以前“什么样”，对应了what。
2.There is much chance ____ Bill will recover from his injury in time for the race.
A. that B. which C. until D. if
答案A。that引导的名词从句陈述事实，说的即是前面的chance，为同位语从句。
三、定语从句与同位语从句中that的区别，主要看that在从句中有没有语法作用。
定语从句中的关系代词that在句中做宾语或主语，有时可以与 which 互换。同位语从句中的连词that只起连接作用，在从句中不担当任何句子成分；而如果句子是同位语从句，就应用连词that而不能用which。同位语从句一般放在表具体含义的名词如：fact、suggestion、truth、possibility、idea等后面，解释名词的含义或内容，而定语从句只是对先行词的限定和修饰。
例如：
The football star Henry expressed the hope that he would come to visit China again in 2008.
同位语从句，从句就是hope的内容；
The hope the football star Henry expressed is that he would come to visit China again in 2008.
定语从句，从句修饰hope，是“表达的”愿望。
再如：
1.…It's thirty years since we last met.
…But I still remember the story, believe it or not, ____ we got lost on a rainy night.
A. which B. that C. what D. when
答案为B。同位语从句，解释story。
2.I still remember the big names in the stories ______ my mother told me when I was young.
A. when B. that C. what D. where
答案为B。定语从句，意指stories，做told的宾语。