问题补充：
He is a man of great knowledge,from whom much can be learned这里的much作什么用?
答案：
much是从句的主语
from whom much can be learned是从句
from whom在从句中作状语
can be learned在从句中作谓语
======以下答案可供参考======
供参考答案1:
It is a noun, used as a subject.
供参考答案2:
这里的much是不定代词 作定从中的主语
