There are different rules,which it would be dangerous to ignore中it为什么这么用

it作为形式主语，指代后面的ignore这个动作
it would be dangerous to ignore可以变换一下语序
即：to ignore (the rules) would be dangerous

补充：which作为定语从句的先行词，指代的是前面的the rules

it是形式主语代指the rules。既然是形式没有实际意义。不论单数复数都用it代的。就是个形式而已。

which是非限制性定语重句用的，简单的说就是定语重句在逗号后面用WHICH而不用that