that从句作为宾语从句的时候,that本身是没有任何含义的.
如：I regret much that I have hurt you.我非常后悔我伤害了你.that在这边是没有意思.也不像定语从句中它要指代一个名词的.同时在动词后面的宾语从句中,that还可以省略的.I regret much （that） I have hurt you.
what = something that
I regret much what I have done to you.= I regret much something that I have hurt you.此时的what还有自己的含义,表示事情,还作为done的宾语.
你可以理解下,有问题我们再交流下