一、引导主语从句
that与what都可引导主语从句，that在从句中不作句子成分，但不能省略。what 在从句中可作主语、宾语、表语等，不能省略。如：
What he said at the meeting was very important．他在会上所说的非常重要。（宾语）
What surprised us is that he did it alone．使我们吃惊的是他一个人做了那项工作。（主语）
That he did it alone surprised us．他独自完成了那项工作使我们感到吃惊。（that用来引导主语从句，在句中不作成分）
二、引导宾语从句
that与what都可引导宾语从句，that在从句中不作句子成分，一般情况下可以省略，但当有两个以上的宾语从句时，只能省掉第一个that。what 在从句中可作主语、宾语、表语等，不能省略。如：
They stopped to see what was happening．他们停下来看发生了什么事情。（主语）
The teacher got angry because of what the boy had done．那个男孩子做的事情让老师很生气。（what在宾语从句中作宾语）
All the people believed that it was right to rescue the temple．所有的人都认为拯救古庙是正确的。（that在宾语从句中不作句子成分且可省略）
We must believe that each one of us is able to do something well and that when we discover what this something is we must work hard at it until we succeed．我们必须相信，我们每一个人都能把某件事情办好；而且，当我们发现这事情是什么的时候，我们就必须努力干下去，直到成功为止。（第一个that可省略，第二个that不能省略）
三、引导表语从句
that与what都可引导表语从句，that在从句中不作句子成分，且不能省略；what 在从句中可作主语、宾语、表语等，不能省略。如：
Mary is no longer what she used to be．玛丽不再是从前那样了。（what在表语从句中作表语）
The question is what we should do to prevent him from going．问题是我们怎样做才能阻止他去。（what在表语从句中作动词do的宾语）
The reason why he didn't come today is that his mother is ill．他今天没来的原因是他母亲病了。（that只起连接作用）
四、引导同位语从句
that与what都可引导同位语从句修饰表示抽象概念的名词，对所修饰的名词的具体内容进行详细阐述。这类名词有：fact, idea, news, hope, belief, promise, thought, suggestion, proposal等。that在从句中不作句子成分，但不能省略。what 在从句中可作主语、宾语、表语等，不能省略。如：
I have no idea what we should do next．我不知道我们下一步该做什么。（宾语）
He made a promise that he would study hard．他许诺要努力学习。(that只起连接作用)
The boy expressed his hope that he would be a pilot when he grows up．那个男孩儿表达了他长大后想当一名飞行员的愿望。（that只起连接作用）
五、引导定语从句
that可引导定语从句且在定语从句中作主语或宾语，that既可指人也可指物。that在从句中作宾语时可省略。如：
This is one of the buildings that were built last year．这是去年建的楼房之一。（that在定语从句中作主语，指物，不能省略。）
The doctor that you met in the street yesterday came here 20 years ago．你昨天在街上遇到的那位医生二十年前就来到这里了。（that 在从句中作宾语，指人并可省略。）
注意：what不能引导定语从句。如不能说：
1．He did all what he could to save the patient．
2．All what he needs is more time．
应将句中的what改为that，或去掉what，还可以将以上两句中的all去掉，但两句的结构发生了根本的改变：句1中的what引导宾语从句，句2中的what引导主语从句。
六、引导状语从句
so．．．that, such ..．that, so that结构引导状语从句。what不能引导状语从句，但whatever, no matter what可引导让步状语从句。如：
He made such rapid progress in English that before long he could write articles for an American newspaper．他的英语进步很快，不久就能为美国的一家报社写文章了。
Put the little plants in the shade so that the sun won't burn them．请把幼苗放在阴凉处以免太阳晒枯它们。
Don't believe him no matter what he says．无论他说什么，都不要相信他。
