關係代詞 as 用於引導非限制性定語從句時，應注意以下三點：
一. as引導的非限制性定語從句位於句末時，一般可以和which互換。
the author was brought up in a small village, as (which) is recounted in some of his stories.
作者是在一個小村莊裡長大的，這個問題在他的一些小說了就被提到過。
she is extremely popular among the students, as (which) is common knowledge.
她在學生中間很受歡迎，這是普遍知道的。
但是，當主句是否定句時，as和which就不能互換，因為as引導的非限制性定語從句不包含否定意義。
spiders are not insects, as many people think.
蜘蛛不是昆蟲，而許多人卻認為蜘蛛是昆蟲。
二. as引導的非限制性定語從句可以放在句子的最前面，而which引導的定語從句不能放在句首。
as is often the case, the girl forgot to her dictionary.（不能用which）
經常是這樣子的，那個女孩忘了帶字典。
as you will find out, all is now settled.
你將會看到這樣的情況，一切都已搞定了。
三. as引導的非限制性定語從句可以放在句子的中間，which引導的定語從句也可以放在句子的中間。
water, which is a clear liquid, has many uses.
水是一種透明的液體，有許多用途。
taiwan is, as you know, an inseparable part to china.
眾所周知，台灣是中國不可分割的一部分。