A weaker requirement would be that the theory must provide a practical and mechanical method for determining whether or not a grammar proposed for a given corpus is, in fact, the best grammar of the language from which the corpus is drawn.

句子大意是：较低的要求是，该理论必须提供一种实用而机械的方法，用来确定用于所给语料库的语法，是否真正为最好的语言规则，该语料库就是取材于这样的语言。

from which 引导定语从句，修饰the language，即先行词是the language，be drawn from意为“来自，取材于”。