同学你好，该知识点来自沪江网校《新概念英语1、2、3、4册连读【学霸班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~
it形式主语。if it were not for…若非;要不是
if it were not for the protection we get from insect-eating animals 要不是人类受一些食虫动物的保护

as引导的非限定性定语从句
主句具有否定意义，as引导的定语从句位于主句之后时，as引导的从句不包括否定意义，意思可用but取代。
Whales are not fish, as some people think / but some people think they are.
I am no orator, as Brutus is.我不善言词,而勃鲁特斯却是个雄辩家.
Spiders are not insects, as many people think, nor even nearly related to them.
蜘蛛不是昆虫,尽管许多人认为它是昆虫,它甚至与昆虫几乎没有关系.

但as引导的定语从句位于主句前时，as指代的主句仍包括否定意义。
As has been said above, grammar is not a set of dead rules.正如前面所说的，语法不是一套死条文。

祝同学学习进步~