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The boys who are playing football are from Class One.正在踢足球的男孩们是一班的。
The man（whom/who/that） you met just now is my old friend.你刚才遇见的那个人是我的老朋友。
The number of people that/who come to visit the city each year reaches one million. 每年来游览这座城市的人数达到一百万。
I once lived in the house whose windows were much larger than usual.我曾经住在那个窗户比一般窗户大很多的房子里。
① as 引导限制性定语从句
as 既可指人也可指物。在定语从句中可作主语或宾语。它常用在such...as...，the same...as... 等结构中，as 不能省略。
Such reasons as we give can persuade him to give up this foolish plan.我们给出的这些理由可以说服他放弃这个愚蠢的计划。
②as 引导非限制性定语从句
as引导非限制性定语从句，代替整个主句的内容。从句可放在主句前、主句后或主句中间。
as 引导非限制性定语从句常用的固定表达：
as we all know 众所周知 as is well known 众所周知
as you see 如你所见 as we expect 正如我们预料的那样
as often happens 正如经常发生的as I can remember 正如我所记得的
as has been said before 如前所述as is/was expected 正如预料的那样
as can be seen 正如所见 as is often the case 情况常常是这样
As is often the case，girls like dolls while boys like guns.
女孩喜欢玩具娃娃而男孩喜欢枪，这是常见的情况。
关系代词as 与which 引导的非限制性定语从句，其先行词都可以是整个主句，as/which 指代整个主句的意思。但as 和which 具有不同的词义、句法和用法。
①as 引导的定语从句表示说话人的看法、态度、解释或评论。as仍具有“正如，像，由……可知”等意思，翻译时有时可不必译出。as 引导的此类从句可置于句首、句中或句尾。
As Napoleon once said，attack is the best method of defense.拿破仑曾经说过，进攻是最好的防御。
②which 引导的定语从句对主句所叙述的事情进行补充说明，表明事物的状态或结果。which 此时指前面主句所提到的这件事，常译为“这一点，这件事”等。这时它引导的从句与主句之间常含有并列、因果关系。它引导的从句的位置不像as 引导的从句那样灵活，只能位于主句的后面。
He changed his mind again，which（= and that） made us all angry.
他又改变了主意，这使我们大家都生气了。
