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非限定性定语从句起补充说明作用，在句子中不充当成分，缺少也不会影响全句的理解，它与主句往往用逗号隔开。定语从句有限制性和非限制性两种。
限制性定语从句是先行词不可缺少的部分，去掉它主句意思往往不明确;非限制性定语从句是先行词的附加说明，去掉了也不会影响主句的意思，它与主句之间通常用逗号分开，将非限定性定语从句放在句子中间，其前后都需要用逗号隔开。
非限定性定语从句的分类
who引导
Our guide，who was a French Canadian，was an excellent cook.
我们的向导，一个法裔加拿大人，是一位优秀的厨师。
My gardener，who is very pessimistic，says that there will be no apples this year.
我家的园丁非常悲观，他说今年将不结苹果。
whom引导
关系代词whom用于指人，在句中作动词宾语和介词宾语，作介词宾语时，介词可位于句首。
如:Peter, whom you met in London, is now back in Paris.
彼得现在回巴黎了，你在伦敦见过他。
Mr Smith，from whom I have learned a lot，is a famous scientist.
史密斯先生是一位著名的科学家，我从他那儿学了许多东西。
whose引导
whose是关系代词who的所有格形式，在从句中作定语。whose通常指人，也可指动物或无生命的事物。
如:The boy,whose father is an engineer, studies very hard.
这个男孩，他的爸爸是一位工程师，学习很努力
Above the trees are the mountains, whose magnificence the river faithfully reflects on the surface.
在树林的高处是山，其壮丽的景色完全映照在河面上。
The play，whose style is rigidly formal，is typical of the period.
这剧本是那个时期的典型作品，风格拘谨刻板。
which引导
关系代词which在非限制性定语从句，中所指代和修饰的可以是主句中的名词、形容词、短语、其他从句或整个主句，在从句中作主语、动词宾语、介词宾语或表语。
①which指代主句中的名词，被指代的名词包括表示物、婴儿或动物的名词、表示单数意义的集体名词以及表示职业、品格等的名词。
如:These apple trees，which I planted three years ago，have not borne any fruit.
这些苹果树是我三年前栽的，还没有结过果实。
She is an artist，which I am not.
她是一位艺术家，而我不是。
Water，which is a clear liquid，has many uses.
水是一种清澈的液体，有许多用途。
The two policemen were completely trusted，which in fact they were.
那两个警察完全受到信任，事实上也确实如此。
②which指代主句中的形容词。
如:She was very patient towards the children，which her husband seldom was.
她对孩子们很耐心，她丈夫却很少这样。
She is always careless，which we should not be.
她总是马虎大意，我们可不应该这样。
③which指代主句中的某个从句。
如:He said that he had never seen her before，which was not true.
他说以前从没见过她，这不是真的。
④which指代整个主句。
如:In the presence of so many people he was little tense, which was understandable.
在那么多人面前他有点紧张，这是可以理解的。
He may have acute appendicitis，in which case he will have to be operated on.
他可能得了急性盲肠炎，如果是这样，他就得动手术。
When deeply absorbed in work，which he often was，he would forget all about eating and sleeping.
他经常聚精会神地工作，这时他会废寝忘食。
when引导
关系副词when在非限制性定语从句中作时间状语，指代主句中表示时间的词语。
如:He will put off the picnic until May 1st, when he will be free.
他将把郊游推迟到5月1号，那时他将有空。
where引导
副词where在非限制性定语从句中作地点状语，指代主句中表示地点的词语。
如:They went to London，where they lived for six months.
他们去了伦敦，在那儿呆了六个月的时间。
as引导
as引出非限定性定语从句时，代替整个主句，对其进行说明但通常用于像as we all know, as it is known, as is known to all, as it is, as is said above, as always mentioned above, as is usual, as is often the case, as is reported in the newspaper等句式中。as在非限定性定语从句中作主语、宾语，且引出的从句位置比较灵活，可位于句首或句末，也可置于主句中间。通常均由逗号将其与主句隔开。as有"正如……，就像……"之意。
如:As is known to the United States, Mark Twain is a great American writer.
美国人都知道，马克·吐温是一位伟大的美国作家。(as在从句中作主语)
He forgot to bring his pen with him, as was often the case.
他忘了带笔，这是常事。(as在从句中作主语）
He is absorbed in work, as he often was.
他正在全神贯注地工作，他过去经常这样。(as在从句中作表语)
As we all know, the earth is round.
众所周知，地球是圆的。(as在从句中作宾语)
The two brothers were satisfied with this decision，as was agreed beforehand.
两兄弟对此决定都满意，这项决定在事前都已得到他们的同意。(as在从句中作主语)
Taiwan is，as you know，an inseparable part of China.
你知道，台湾是中国不可分割的一部分。(as在从句中作宾语)
as it is known to all (that...)(as 做状语从句连词，是状语从句, 可以加that)
与 as is known to all(as做主语，是定语从句)
与 it is known to all that...(主语从句，it是形式主语)
即:As is widely known, the moon is closer to the us than the sun.(定从)
= That the moon is closer to the us than the sun is widely known.(主从)
= It is widely known that the moon is closer to the us than the sun.(主从)
As it is known to everyone, I thought you knew about it too.(状从)
关系代词which有时并不代表主句中某一确定的词，而是概括整个主句的意思。介词的选择取决于它与先行词的搭配或与从句中谓语动词的搭配。
They were short of sticks to make frames for the climbing vines，without which the yield would be halved.
他们缺搭葡萄架的杆儿，没有它们产量会减少一半。
They thanked Tom，without whose support they would not have succeeded.
他们很感激Tom，没有他的支持他们是不会成功的。
"介词+关系代词"引导非限制性定语从句:在介词后引导非限制性定语从句。
非限定性定语从句的特殊结构
"名词/代词+of+which/whom/who "
It now has 20，000 hectares of land，more than two-thirds of which are under cultivation.
现在它拥有两万公顷土地，其中三分之二之多已经耕种。
Light is the fast thing in the world, the speed of which is 300000 kilometer per second.
光是世界上最快的东西，它的速度是每秒30万千米。
There are 30 chairs in the small hall, most of which are new.
大厅里有三十把椅子，绝大部分是新的。
The textile mill has over 8，000 workers and staffs，eighty per cent of who are women.
这家纺织厂有8千多职工，女职工占百分之八十。