knows
A teacher：no matter how much he knows can't teach his students everything they want to know
Snape is now establishing just how much Harry knows about the incident.
斯内普现在在问哈利对这件事情知道多少.
learns 是指学会了,有终于知道了意思
如：
He learns that how much money a family has is not important.
他终于知道家里有没有钱并不重要.
本题的翻译：无论老师的知识有多丰富,也不能回答学生提出的每个问题.