1)what = the thing which;whatever = anything.例如：

What you want has been sent here. 你要的动词都送来了。

Whatever you want makes no difference to me. 不管你要什么，跟我没什么关系。

2) who= the person that whoever= anyone who.例如：

(错)Who breaks the law will be punished.

(错)Whoever robbed the bank is not clear.

(对)Whoever breaks the law will be punished. 王子犯法，与庶民同罪。

(对)Who robbed the bank is not clear. 谁抢了银行还不清楚。

3) that 和 what

当that引导定语从句时，通常用作关系代词，而引导名词性从句时，是个不充当任何成分的连接词。宾语从句和表语从句中的that常可省略。What只能引导名词性从句，用作连接代词，作从句的具体成分，且不能省略。例如：

I think(that)you will like the stamps. 我想你会喜欢这些邮票的。

What we need is more practice. 我们需要的是更多的实践。