1)what = the thing which;whatever = anything

What you want has been sent here.

Whatever you want makes no difference to me.

2) who= the person that whoever= anyone who

(錯)Who breaks the law will be punished.

(錯)Whoever robbed the bank is not clear.

(對)Whoever breaks the law will be punished.

(對)Who robbed the bank is not clear.

3) that 和 what

當that引導定語從句時 ，通常用作關系代詞，而引導名詞性從句時，是個不充當任何成分的連接詞。賓語從句和表語從句中的that常可省略。What只能引導名詞性從句， 用作連接代詞，作從句的具體成分，且不能省略。

I think (that) you will like the stamps.

What we need is more practice.