关系代词as与which引导的非限制性定语从句，其先行词都可以是整个主句，指代主句的整个意 思。但as和which具有不同的词义、句法和用法。
1. as引导的从句表示说话人的看法、态度、解释或评论
引导定语从句时，as仍具有“正如，像，由……可知”等意思，翻译时 有时可不必译出。as定语从句常含有这些动词：see, know, hear, watch, remember, say, tell, show, expect, guess等，这类动词与as连用几乎成了一种固定搭配。as引导的此类从句可以置于句首、句中或 句尾。如：
The famous magician Liu Qian, as we all know, is from Taiwan. 众所周知，著名魔术师刘 谦是台湾人。
There is a net bar around here, as I remember. 我记得这儿附近有一家网吧。
As is often the case, girls like dolls while boys like guns. 女孩子喜欢玩具而男孩子 喜欢枪，这是常见的情况。
2. which引导的从句对主句所叙述的事情进行补充说明，表明事物的状态或结果
which此时指前面主句所提到的这件事，常译为“这一点，这件事”等。这时它所引 导的从句与主句之间常含有并列、因果关系。注意，它引导的从句不像as那样位置灵活，它只能位于主句 的后面。如：
He changed his mind again, which made us all angry. 他又改变了主意，这使我们大家都生 气了。
She tore up my photos, which (=and that) upset me.她撕碎了我的照片，这使我很不安。
3. 在从句中作定语或介词的宾语时，要用which，不用as
Jenny might come, in which case I'll ask her. 詹妮可能会来，要是那样的话我就去问 她。
She graduated from a local middle school, after which she went to Beijing University. 她毕业于当地的一所中学，之后她去了北京大学。
4. 当从句的谓语动词是否定形式或含着一个复合宾语时，一般用which而不用as
He pretended not to know me, which I didn't understand. 他假装不认识我，这是我搞 不明白的。
He admires everyone in his class, which I find quite strange. 他羡慕班里的每个学生， 我认为太奇怪了。
【特别说明】
as引导非限制性定语从句的最大特点是位置灵活;其次，as有以下常见的，而which不具备的 “固定表达”：
as we all know 众所周知
as I can remember 正如我所记得的
as often happens 正如经常发生的那样
as we expect 正如我们预料的那样
as you see 这一点你明白
as was expected 正如预料的那样
as can be seen 看得出来
as may be imagined 正如可以想象的那样
as has been said above 如上所述
as we all can see 正如我们都能看到的那样
as is often the case 像常规那样
as everybody can do 正如人人都能做到的那样
