引导词as可引导限制性定语从句,也可引导非限制性定语从句。
1.在固定词组the same...as,such...as中,as引导限制性定语从句。如:
He is not such a fool as he looks.
I've bought the same dress as she is wearing.
2.当先行词是整个主句时,可用 as /which引导非限制性定语从句。引导词as和which的区别在于:
①as引导的非限制性定语从句可放于主句前、主句中、主句后,而which引导的非限制性定语从句一般放于主句后或句中。
②as常与从句中的know,see, hear,expect等动词连用,也常用于as often happens,as is often the case(常有的事)等句子中;而which一般不用于以上情况。
③as有“正如”的含义,which没有此含义。如:
As is well known,Shanghai is a big city.
Some of the roads were flooded, which made our journey more difficult.
关系代词who,whom,whose, which和as能引导非限制性定语从句,而that不能;非限制性定语从句中即使缺宾语,引导词也不能省略;关系副词when,where能引导非限制性定语从句,而why不能。如:
Abraham Lincoln,who led the United States through the Civil War years,was shot on April 14,1865.