1）what是一个疑问代词,它在句子或从句中的成分为：主语、宾语、表语等
它一般用于主语从句、宾语从句、表语从句.
如 i don't know what happened to him.从句中的主语
i don't know what he did.从句中宾语
i don't know what he is .从句中的表语
2）how在句子或从句中的成分为：状语,从句中将how删除,句子还通顺
如 i don't konw how he finished the work.从句中作状语,将从句how he finished the work中的how删除剩下的 he finished the work这个句子仍然通顺.而what则没这个用法.