1. 以下两个句子，是有些网友犯错的句子：

I'll do all what I should do.
This is the way how Tom did the work.

错句改正：

I'll do all that I should do. 或 I'll do what I should do. 

This is the way that (in which) Tom did the work. 或 This is how Tom did the work.

2. what 不能直接作关系代词，因为它本身是一个融合性关系代词，身兼两职：先行词 + 关系代词 = the thing(s) that。例如：This is what he said.

3. how 不能直接作关系副词，因为它本身是一个融合性关系副词，身兼两职：先行词 + 关系副词 =（in） the way that (in which) 。意为：以任何方式，无论用什么方法。例如：You can answer the two historical questions how you like.

这就是：定语从句的关系代词为什么不能用what、关系副词为什么不能用how 的原因。