这句话的主语是什么?but引导的分句的主语是什么?ItisimpossibletoknowtodayjustwhatourStoneAgeancestorsknewaboutplants,butfromwhatwecanobserveofpreindustrialsocietiesthatstillexistadetailedlearningofplants

Question

这句话的主语是什么?
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.

Answer 1

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