由as,which引导的非限定性定语从句,相当于and this或and that。在很多时候,表示说话人对话语的看法或态度,此时它们的先行词并不是具体的某个词,而是整个句子或短语。as,which用在句末的时候,有些情况下可以互换使用。但是,当主句是否定句时,只能as。
(1) 当先行词前面有so + adj. + a / such + (a) + adj. , the same等词修饰时,引导词只能用as,而不用which。如:
She is so nice a girl as we all like. 她是个我们大家都喜欢的如此好的女孩。
It is such an interesting book as we all want to read. 它是本我们大家都想读的有趣的书。
上面两句中的as就不能用which替换。
(2) as 在从句中常作宾语;作主语时,从句的谓语动词常为系动词,即:系表结构或被动结构。如:
As you expected, he turned up on time. 正如你所愿,他按时到了。
As is said above, he has conquered the difficulty. 正如上面所说,他征服了困难。
(3) 当非限制性定语从句放到主句的前面时,引导词只能用as,而不用which。如:
As is planned, we got there before eight. 正如计划,我们再把店前到达了那儿。
(4) As引导的非限制性定语从句指表示积极的意义,与主句是顺理成章的关系,绝不能与之矛盾。若非限制性定语从句表示一种消极的意义,则只用which。如:
He has succeeded in his career, as / which we all hope. 他在事业中成功了,这正是我们大家所希望的。
Her grandma died last week, which made her very sad. 她的奶奶逝世了,这使得她很悲伤。
(5) 当非限制性定语从句的引导词前面有介词时,只能用which, 而不可用 as。如:
He has a new computer, for which he paid nearly ten thousand yuan. 他有一台新电脑,这台电脑花了他将近一万元。