这句话的主语是什么？

来源：百度知道 编辑：UC知道 时间：2024/06/01 05:21:45
but引导的分句的主语是什么？
It is impossible to know today just what our Stone Age ancestors knew about plants, but from what we can observe of preindustrial societies that still exist a detailed learning of plants and their properties must be extremely ancient.

But:
From what we can obsertve of ---状语从句
Preindustrial sovieties ---主语
that still ..... their properties --- 定语从句
must be .... ancient --- but分句中的主句的下半部分

to know ...

动词不定式是真正的主语，it 是形式主语