As 和which都可以引导非限定性定语从句，在用法上有一些区别。
和which都可以在定语从句中作主语，宾语，既可修饰一个词也可修饰前面的整个句子。
accepted his proposal, as(which)was natural.她接受了她的求婚，这是自然而然的。
boy is pretty handsome, as(which)we can see in our classroom.这个男孩非常的帅气，正如我们在教室看到。
as 在非限定性定语从句中的使用
2.1 as可以放在主句之前，主句之后，也可以放在句中，可以说位置很灵活。但是which常用在主句之后
we know,technology just like a engine to push the development of economics.众所周知，技术就像是引擎推动着经济的发展。
diva,as you know, is famous throughoutthe world.这个女歌剧演唱家，正如你知道的，在全世界都出名。
has stood her up, which I don`t believe.他没有赴约，这事我不相信。
2.2 as与the same，such 连用时，用as。
is the same phone as I have borrowed from my sister.这个手机和我向我姐姐借的.一样。
B.I have never had such a chance to make a speech as I always imagined.我从没有这样一个机会去演讲，正如我一直想象的那样。
2在从句中谓语动词为被动语态，缺少主语时用as，不用which。
yone knows that she has left, as was expected.所有人都知道她又一次离开了，正如我们预料的那样。
被修饰的词叫先行词。定语从句不同于单词作定语的情况，它通常只能放在被修饰的词（即先行词）之后。定语从句由关系词（关系代词、关系副词）引导，关系代词、关系副词位于定语从句句首。关系代词引导的定语从句 关系代词所代替的先行词是人或物的名词或代词，并在句中充当主语、宾语、定语等成分。关系代词在定语从句中作主语时，从句谓语动词的人称和数要和先行词保持一致。
1）who, whom, that 这些词代替的先行词是人的名词或代词，在从句中所起作用如下： Is he the man who/that wants to see you?他就是想见你的人吗？（who/that在从句中作主语） He is the man whom/ that I saw yesterday.他就是我昨天见的那个人。（whom/that在从句中作宾语）
2) Whose 用来指人或物，(只用作定语, 若指物，它还可以同of which互换）, 例如： They rushed over to help the man whose car had broken down. 那人车坏了，大家都跑过去帮忙。 Please pass me the book whose (of which) cover is green. 请递给我那本绿皮的书。
3）which, that 它们所代替的先行词是事物的名词或代词，在从句中可作主语、宾语等，例如： A prosperity which / that had never been seen before appears in the countryside. （which / that在句中作主语） 农村出现了前所未有的繁荣。
