(五)关系代词as引导的定语从句
关系代词as既可以引导限制性定语从句，也可以引导非限制性定语从句，as在从句中作主语、宾语或表语。
引导限制性定语从句常用于下列形式：
名词+as像一样的，像之类
名词+as 和同样的
其中关系代词as在从句中担当主语、宾语或表语。如：
We have found such materials as are used in their factory.
He is not the same man as he was.
2.such as
Such为代词，意为“这样的人或物”，as在从句中担当成分，修饰先行词such.如： This book is not such as I expect.
(六)关系代词as, which的区别
1.as引导的非限制性定语从句，既可在主句前，又可在主句后，有时还克插入主句中，而which引导的非限制性定语从句只能置于主句之后。相同的是两者都可以替代主句的整个内容，而不是主句中的某一个词。如：
The weather turned out to be very good, which was more than we could expect. By serving others, a person focuses on someone other than himself or herself, which can be very eye-opening and rewarding.
2. 当非限制性定语从句放在主句前面时，只能用as.如：
As is known to everybody, the moon travels round the earth once every month. = The moon travels round the earth once every month, as/which is known to everybody. = It is known to everybody that the moon travels round the earth once everybody. = What is known to everybody is that the moon travels round the earth once every month.
后面两句属于名词性从句范畴。
另外，as多用于下列习惯用语中：as anybody can see 正如人人都能看到的那样;as is well-known=as is known to all众所周知;as we had expected正如我们所预料的那样;as often happens 正如经常发生的那样;as has been said before 如上所述;as is mentioned above正如上面提到的。
3.当定语从句放在主句后面是，也并不是as就能永远等于which.
(1)当限定性定于从句是否定句或表示否定时只能用which.如：
He came here very late, which was unexpected.
Mr. Smith usually praises his students Rose in public, which she doesn’t like at all.
(2)当as在从句中作主语时，后面常接动词的被动语态。如：be known, be said, be reported, be announced等。如果从句中行为动词是主动语态，一般用which作主语。如： She has been absent again, as is expected. 她又缺席了，这在预料之中。 Tom has made rapid progress, which makes me very happy. Tom取得了很大的进步，这使我很高兴。
(3)用在as(it) seems likely, as(it) often happiness, as(it) was printed out, as(it) was said earlier, as I remember (it), as I understand(it), as (it) appears等结构中。如：
Jack has won the first prize, as it often happens.
Jack和往常一样，获得了一等奖。
She has read widely in Romantic literature, as it appears from her essay. 她广泛涉猎了浪漫主意文学，这从她的文章中可以看出来。
(4)as仍然保持作连词时常有的某种含义。如：
David is tall, as are my brothers.
David很高，我的兄弟也一样。
He opposed the idea, as could be expected.
不出所料，他反对这个意见。
(5)当非限定性定语从句的谓语是一个复合结构时，只能用which引导定语从句。如:Betty always tells a lie, which her parents find strange.
(6)当非限定性定语从句为否定句时，常用which.如：
Mr. Smith usually praises his student Rose in public, which she doesn’t like at all.
三、关系副词的用法
(一)当先行词在定语从句中作状语时，要用关系副词。其中when=表时间的介词(如：ai, in, during等)+which; where=表地点的介词(如：in, at, on, under等)+which; why=表原因的介词(如：for)+which; how=表方式的介词(如：in)+which。如：
I still remember the day when I first came to Beijing.(when=on which)
Can you tell me the office where he works? (where=in which)
Do you know the reason why he is absent?(why=for which)
(二)介词+关系代词(which)=where/when.有时候为表达清楚，还可以在关系副词where/when前加介词from, to等。如：China is the birth place of kite, from where kite flying spread to Japan, Korea, Thailand and India.
(三)对关系副词where的考察
对于where的考察趋于复杂，从先行词为明显的“地点”转化为“地点的模糊化”。事实上，对于where这个词，不能只理解为表示地点。先行词表示某人或物的situation，或者某事所发展的stage，或表达某事的某个方面时，都可以用where，这个关系副词，如：
The accident had reached to a point where both their parents are to be called in. 事情发展到如此程度，不得不请双方家长来一趟了。
这种用法不仅仅限于定语从句，特殊疑问句中、名词性从句中也有如此用法。
四、关系副词和关系代词的`比较
引导定语从句的关系代词及关系副词除了起连接先行词和从句的作用外，他们还有一个最重要的作用，那就是他们分别在定语从句中担当成分。具体地说，关系代词在定语从句中作表语、主语、宾语，而关系副词在定语从句中做状语。因此，在选择关系时，最重要的是分析一下定语从句中的成分，若从句中却主语、宾语或表语，那么必须用关系代词，若主句中不缺主语、宾语或表语则用关系副词。
试比较下面的句子：
(1)、Do you still remember the days that/which we spent in Beijing?(代)
(2)、Do you still remember the days when we spent the summer holidays in Beijing?(副)
在句中，定语从句中缺宾语，第二局中，定语从句不缺宾语也不缺主语，因此用关系副词when来引导。
五、定语从句用法其他要点
(一)、关系代词在定语从句中作主语时，不可省略。
(二)、定语从句中的谓语动词与先行词保持一致;当引导定语从句的关系代词在从句中作主语时，那么定语从句中的谓语动词在人称、数的方面，应该与先行词保持一致。
1.one of + 复数名词+关系代词+复数动词。如：
The Great Wall is one of the world-famous buildings that draw lots of visitors.长城是吸引大批游客的世界著名的建筑之一。
Titanic is one of the wonderful movies that have been produced in Hollywood. 《泰坦尼克号》是好莱坞生产的最精彩的电影之一。
2.the only one of+复数名词+关系代词+单数动词。如：
The Great Wall is the only one of the buildings on the earth that is seen from the moon.长城是地球是唯一一个能从月球上看到的建筑物。
Titanic is the only one of these wonderful movies that has been produced in Hollywood. 在这些精彩的电影中，《泰坦尼克号》是唯一由好莱坞制作的电影。
注意：not the only one of=one of如：
Tom isn’t the only one of the boys who have passed the exam.
=Tom is one of the boys who have passed the exam.
Tom 并非是唯一一个通过考试的男孩=Tom只是通过考试的男孩中的一个。
3.当关系代词as与which引导非限制性定语从句，修饰主句内容时，若as与which作主语，则从局的谓语动词用单数形式。如：
Great changes have taken place in China, as is known to all.
He has passed the college entrance examination, which makes his parents quite happy.
4.其他情况
I, who am your teacher, will try my best to help you.
To own a computer in families, which we thought was impossible twenty years ago, now becomes true.
(三)、注意way和time后接定语从句的情况
1.当先行词是way意味“方式、方法”时，引导定语从句的关系代词有下列三种形式 不填
他向我们解释的那种方法很简单。
The which he explained the sentence to us was not difficult to understand. 不填
他向我解释句子的那种方法不难理解。
2.先行词是time,若time作“次数”讲时，应用关系代词that引导定语从句，that可省略;若time作“一段时间”讲时，应用关系副词when或介词at/during+which引导定语从句。如：This is the second time (that) the President has visited the country. I could hardly remember how many times (that) I have failed.