英语:定语从句中什么时候用As作为先行词的关系词?

1、在“the same…as”和 “such…as”结构中,as是关系代词,引导非限制性定语从句,as在从句中作主语、宾语或表语.如：
⑴．Is it the same tree as can live everywhere?
⑵．I live in the same city as he (live in)．
⑶．China is no longer the same as it used to be．
⑷．I wish there would be such a machine as can work out all the problems．
⑸．We hope to get such a tool as he is using．
⑹．Such man as he is isn’t dealt with．
2、as引导的非限制性定语从句,可以用来指代整个主语或主语的一部分内容,在这种情况下,as和which有时可以交换使用,有时又不可以交换.除了as引导的非限制性定语从句可放主语之前而which引导的从句不可以外,as的用法还有下列几种情况,以区别于which.
（1）as引导的非限制性定语从句,对主句的内容起积极的作用,反之则
应用which．使比较：
Mother gave him some money,as was a pleasure to him．
母亲给他些钱,这使他很高兴.
Mother gave him some advice,which made him unhappy．
母亲给他提了点建议,这使他很不高兴．
（2）、强调成分不同.As强调主句内容,which强调从句内容.如：
Her speech was wonderful,as we had expected．
正如我们所期盼的那样,她的发言棒极了.
His father died,which made him very sad．
他父亲去世了,这使他很悲伤.
（3）、在非限制性定语从句中,谓语是系词be时,常用as作主语,而谓语是实义动词时,则用which作主语.如：
Tom is late for class,as is often the case．
汤姆迟到了,他往往是这样.
They are hollow,which makes them very light．
都是空心的,这使它们重量很轻.
（4）as引导的非限制性定语从句要与主句语义一致.如果主句为肯定句,从句用否定句,则一般用which引导.如：
Mr.Johnson is a teacher,as is clear from his manner．
约翰逊先生是位教师,这从他的举止可以看出来.
He has to work on Sundays,which he doesn’t like．
他必须在星期天工作,他不喜欢这样.
（5）、as含有“正如……”的意思,常与表示“判断、了解,指出”等意思的动词连用,作这些动词的宾语,或该动词用被动语态,as作主语.
She believed the fact,as had been expected．（as作主语）
As we know now,bats come out only at night．（as作宾语）
这些as引导的,被动结构的非限制性定语从句,已构成固定的形式和固定的译文.（6）、as不能用于“介词+关系代词”结构引导定语从句.如只能说：He finished his work,after which he sat some time reading the newspaper．他完成了他的工作,之后他坐着看了一会儿报纸.Which不能用as代替.