用 which 或 as 引导,至于两者的区别,请参考:
http://www.yygrammar.com/Article/201305/3299.html
http://www.yygrammar.com/Article/201512/4615.html
http://www.yygrammar.com/Article/200809/487.html
先行词是句子时,as 或 which 都可以引导非限定性定语从句,但要注意用法的不同:
(1)as 从句可以放在句首,居于先行词之前,结构上很像状语从句,which 则不能放在句首。
As 【正】Which【误】 was natural, he married her. 他娶了她是很自然的事。
(2)当关系代词是一般动词的主语(非 be 动词)时,只能用 which 。
He saw the girl, as 【误】/ which 【正】delighted him.【as 只能做 be 动词的主语】
He saw the girl, as 【正】/ which 【正】he had hoped (he would).【as 和 which 都可以做动词的宾语】
He married her, as 【正】/ which 【正】as natural.【as 和 which 都可以做 be 动词的主语】
(摘自《刘毅英语语法宝典》p.160)