as可以在非限制性定语从句中做关系代词吗?如果可以它在非限制性定语从句中与which有什么区别?

非限制性定语从句：作用相当于一种插入语或者对先行词的一种解释,和先行词之间只有比较松散的关系,文字中常常用逗号将其与主句分开,用法其实与限制性定语从句极为相似,只是不能用that做修饰词.例句：This letter is from his parents,who are working in Tibet.Englishi is an important subject,which every students should study well.The building,in front of which sat a boy,was a school
as与which引导非限制性定语从句的区别
1.which 引导的定语从句只能位于主句后,但as引导的从句可位于主句前、中、后.如：
David,as you know,is a photograopher.戴维是个摄影师,你是知道的.(不可用which)
Li Ming is late,as is often the case.李明迟到了,这事是经常发生的.(不可用which)
2.as在定语从句中作主语时,从句谓语通常要有be或别的系动词,但which不受此限制.如：
Li Ming was late,which (=and this) made Mr.Zhang very angry.李明迟到了,这件事使张先生非常生气.(不可用as)
3.as 引导的定语从句只表示一个众所周知或意料之中的事,但 which 不受此限制.如：
He married her,which was unexpected.他和她结婚了,这是出乎意料的.(不可用as)
4.as有“正如”之意,而which则意为“这(件事,一点)”,可用and this/that/it来代替.
5.as多用于固定搭配中：as is often the case(这是常有的事),as was expected(不出所料),as often happens(正如经常发生的那样),as is known to all(众所周知),as has been said before(如上所述),as is mentioned above(正如上面所提到的)等.
注：which 代表整个句子,还可用于in which case,at which point,on which occasion等.如：
I may have to work late,in which case I’ll telephone you.我可能得晚点下班,那样我会给你打电话的.

as与which引导非限制性定语从句的区别
1. which 引导的定语从句只能位于主句后，但as引导的从句可位于主句前、中、后。如：
David, as you know, is a photograopher. 戴维是个摄影师，你是知道的。(不可用which)
Li Ming is late, as is often the case. 李明迟到了，这事是经常发生的。(...