—I think what he needs is more practice.
—Yes. That he needs more practice is clear.
我认为他需要的是更多的练习。
是的。他需要更多的练习是很明显的。
1、用法不同
what引导名词从句时，它在从句中要充当句子成分（主语、宾语、表语等）。
that引导名词从句时，它在从句中不能充当句子成分。
2、意思不同
what引导名词从句时，它有词义，表示：什么；所的（东西）。此时
what=the thing（s）that
what=that； which
that引导名词从句时，仅仅是连接引导从句作用，它没有词义。在宾语从句中有时可以省略。
请看经典例句分析：
What caused the accident is still a complete mystery.
什么引起的事故仍然是个彻头彻尾的迷。
What引导一个主语从句，且what在从句中主语。
What we like to do most on holiday is just to visit old churches.
我们在假期最喜欢做的事情就是参观古老的教堂。
What引导一个主语从句，且what在从句中宾语。
What that child need is a good spanking.
那孩子需要的是被狠狠地打一顿屁股。
What引导一个主语从句，且what在从句中宾语。
What I can’t understand is why he has changed his mind.
我不能理解他为什么改变主意。
What引导一个主语从句，且what在从句中宾语。
What I do with this money is not your concern.
我怎样处理这笔款子与你无关。
What引导一个主语从句，且what在从句中宾语。
It was what they had been working so hard to find: pure radium.
这就是他们一直辛勤劳动、孜孜以求的东西：纯镭。
What引导一个表语从句，且what在从句中find的宾语。
That he will help us is certain.
他将会帮助我们，那是肯定无疑的。
That引导主语从句，但在从句中不作任何成分，仅仅是个连词。
That John became a poet may have been due to his mother influence.
约翰成为诗人可能是受了她母亲的影响。
That引导主语从句，但在从句中不作任何成分，仅仅是个连词。
That the earth is round is known.
已知地球是圆的。
That引导主语从句，但在从句中不作任何成分，仅仅是个连词。
再看：
China is no longer what it used to be.
中国已今非昔比。
It is uncertain what side effect the medicine will bring about, although about two thousand patients have taken it.
目前还不确定这种药会带来什么副作用，尽管大约有2000名患者服用了这种药。
What he said at the meeting made us angry.
他在会上说的话使我们很生气。
That he was late made the teacher angry.
他迟到了，这让老师很生气。
One reason for her preference for city life is that she can have easy access to places like shops and restaurants.
她喜欢城市生活的一个原因是，她可以方便地去商店和餐馆等地方。
That the earth travels around the sun is known to all.
地球绕太阳运行是众所周知的。
He doesn’t know what she likes. 他不知道她喜欢什么。
What you say is quite right. 你所说的相当正确。
I believe (that) he will come to see us. 我相信他会来看我们的。
It’s a pity (that) he didn’t finish college. 真遗憾他大学没毕业。
We are glad that we have accomplished another arduous task.
我们都感到高兴因为又完成了一项艰巨的任务。
（引导状语从句，表示原因或理由）
He is working hard that he can catch up with the class.
他努力用功，以便赶上班上同学。
（引导状语从句，表示目的或结果)为了，以至于）
That I could stay at home today!
今天要是我能呆在家里多好呀!
（that引导表示愿望，感叹的从句，主句常省略）
He expressed the hope that we would keep in touch with his firm.
他表示希望我们与他的公司保持联系。
I had no idea that you were here. 我不知道你在这儿。
(that 在名词性从句中可引导同位语从句，但 what 通常不用于引导同位语从句。)
it 作形式宾语时，that引导的宾语从句时，that不能省略。
He has made it clear that he will not give in.
他已明确表示不会让步
连接两个或两个以上的宾语从句，从第二个开始不能省that:
The teacher said (that) Tom was a clever boy and that he liked him.
老师说汤姆是个聪明的男孩，他喜欢他
主句谓语动词与that从句之间有插入语时, 不能省that:
We decided, in view of his special circumstances, that we would admit him for a probationary.
鉴于他的特殊情况，我们决定给他试用机会。