1．as 与 which 引导非限制性定语从句时的区别

意义 as 常用来表示说话人对于某事的态度、解释或评论等，含有"正如......"的意思 which 一般表示某事的状况或结果 位置 as 引导的从句放在主句的前面、中间或末尾 which 引导的从句只能放在主句之后 　　As we all know, Taiwan belongs to China.

　　正如我们所知，台湾属于中国。

　　Allen made much progress in English, which delighted us.

　　艾伦英语取得了很大进步，这使我们很高兴。

　　2．as 引导限制性定语从句，常与 the same, such, as 等连用，在句中常作主语、宾语或表语。

　　3．as 引导非限制性定语从句，常用在一些固定结构中。

　　as is said above 如上所述

　　as is already mentioned above 就跟上边所提到的那样

　　as is known to all 众所周知

　　as is often the case 就跟经常的情况那样

　　as often happens 就跟经常发生的那样

　　as is reported 就跟所报道的那样

　　as you can see 就跟你看到的那样

　　　2

　　①(江苏高考改编)The number of smokers, as is reported, has dropped by 17 percent in just one year.

　　②(山东高考改编)There is no simple answer, as is often the case in science.

　　③(安徽高考改编)A lot of language learning, as has been discovered, is happening in the first year of life, so parents should talk much to their children during that period.

　　三、"介词＋关系代词"引导的非限制性定语从句

　　1．"介词＋关系代词"可以引导限制性定语从句，也可以引导非限制性定语从句。该结构中关系代词指人时用whom，指物时用which，不可用that；关系代词在从句中作定语时用whose。

　　I had told them the reason, for which I didn't attend the meeting.

　　我没有去开会，我已经把理由告诉他们了。

　　He bought the car for more than $20,000, with which his father was very angry.

他花两万多美元买了这辆车，他父亲对此很生气。