非限定性定語從句
非限定性定語從句起補充說明作用，缺少也不會影響全句的理解，它與主句往往用逗號隔開。定語從句有限制性和非限制性兩種。限制性定語從句是先行詞不可缺少的部分，去掉它主句意思往往不明確；非限制性定語從句是先行詞的附加說明，去掉了也不會影響主句的意思，它與主句之間通常用逗號分開，將非限定性定語從句放在句子中間，其前後都需要用逗號隔開。
分類
who引導
Our guide，who was a French Canadian，was an excellent cook.
我們的嚮導，一個法裔加拿大人，擅長於烹調。
My gardener，who is very pessimistic，says that there will be no apples this year.
我家的園丁非常悲觀，他說今年將不結蘋果。
whom引導
關係代詞whom用於指人，在句中作動詞賓語和介詞賓語，作介詞賓語時，介詞可位於句首。
如：Peter, whom you met in London, is now back in Paris.
彼得現在回巴黎了，你在倫敦見過他。
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學家，我從他那兒學了許多東西。
whose引導
whose是關係代詞who的所有格形式，在從句中作定語。whose通常指人，也可指動物或無生命的事物。
如：The boy, whose father is an engineer, studies very hard.
那位父親是位工程師小男孩學習很努力。
Above the trees are the mountains, whose magnificence the river faithfully reflects on the surface.
在樹林的高處是山，其壯麗的景色完全映照在河面上。
The play，whose style is rigidly formal，is typical of the period.
這劇本是那個時期的典型作品，風格拘謹刻板。
which引導
關係代詞which在非限制性定語從句，中所指代和修飾的可以是主句中的名詞、形容詞、短語、其他從句或整個主句，在從句中作主語、動詞賓語、介詞賓語或表語。
① which指代主句中的名詞，被指代的名詞包括表示物、嬰兒或動物的名詞、表示單數意義的集體名詞以及表示職業、品格等的名詞。
如：These apple trees，which I planted three years ago，have not borne any fruit.
這些蘋果樹是我三年前栽的，還沒有結過果實。
She is an artist，which I am not.
她是一位藝術家，而我不是。
Water，which is a clear liquid，has many uses.
水是一種清澈的液體，有許多用途。
The two policemen were completely trusted，which in fact they were.
那兩個警察完全受到信任，事實上也確實如此。
② which指代主句中的形容詞。
如：She was very patient towards the children，which her husband seldom was.
她對孩子們很耐心，她丈夫卻很少這樣。
She is always careless，which we should not be.
她總是馬虎大意，我們可不應該這樣。
③ which指代主句中的某個從句。
如：He said that he had never seen her before，which was not true.
他說以前從沒見過她，這不是真的。
④ which指代整個主句。
如：In the presence of so many people he was little tense, which was understandable.
在那麼多人面前他有點緊張，這是可以理解的。
He may have acute appendicitis，in which case he will have to be operated on.
他可能得了急性盲腸炎，如果是這樣，他就得動手術。
When deeply absorbed in work，which he often was，he would forget all about eating and sleeping.
他經常聚精會神地工作，這時他會廢寢忘食。
when引導
關係副詞when在非限制性定語從句中作時間狀語，指代主句中表示時間的詞語。
如：He will put off the picnic until May 1st, when he will be free.
他將把郊遊推遲到5月1號，那時他將有空。
where引導
副詞where在非限制性定語從句中作地點狀語，指代主句中表示地點的詞語。
如：They went to London，where they lived for six months.
他們去了倫敦，在那兒呆了六個月的時間。
as引導
as引出非限定性定語從句時，代替整個主句，對其進行說明但通常用於像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在非限定性定語從句中作主語、表語或賓語，且引出的從句位置比較靈活，可位於句首或句末，也可置於主句中間。通常均由逗號將其與主句隔開。as有“正如……， 就像……”之意。
如：As is known to the United States, Mark Twain is a great American writer.
美國人都知道，馬克·吐溫是一位偉大的美國作家。（as在從句中作主語）
He forgot to bring his pen with him, as was often the case.
他忘了帶筆，這是常事。（as在從句中作主語）
He is absorbed in work, as he often was.
他正在全神貫注地工作，他過去經常這樣。（as在從句中作表語）
As we all know, the earth is round.
衆所周知，地球是圓的。（as在從句中作賓語）
The two brothers were satisfied with this decision，as was agreed beforehand.
兩兄弟對此決定都滿意，這項決定在事前都已得到他們的同意。(as在從句中作主語)
Taiwan is，as you know，an inseparable part of China.
你知道，臺灣是中國不可分割的'一部分。(as在從句中作賓語)
“介詞+關係代詞”引導非限制性定語從句：在介詞後引導非限制性定語從句。
難：句意相同
as it is known to all (that...) (as 做狀語從句連詞，是狀語從句, 可以加that)
與 as is known to all (as做主語，是定語從句)
與 it is known to all that... (主語從句，it是形式主語)
即：As is widely known, the moon is closer to the us than the sun.(定從)
= That the moon is closer to the us than the sun is widely known.(主從)
= It is widely known that the moon is closer to the us than the sun.(主從)
As it is known to everyone, I thought you knew about it too.(狀從)[1]
關係代詞which有時並不代表主句中某一確定的詞，而是概括整個主句的意思。介詞的選擇取決於它與先行詞的搭配或與從句中謂語動詞的搭配。
They were short of sticks to make frames for the climbing vines，without which the yield would be halved.
他們缺搭葡萄架的杆兒，沒有它們產量會減少一半。
They thanked Tom，without whose support they would not have succeeded.
他們很感激Tom，沒有他的支持他們是不會成功的。
特殊結構
“名詞/代詞+of+which / whom”
It now has 20，000 hectares of land，more than two-thirds of which are under cultivation.
現在它擁有兩萬公頃土地，其中三分之二之多已經耕種。
Light is the fast thing in the world, the speed of which is 300000 kilometer per second.
光是世界上最快的東西，它的速度是每秒30萬千米。
There are 30 chairs in the small hall, most of which are new.
大廳裏有三十把椅子，絕大部分是新的。
The textile mill has over 8，000 workers and staff，eighty per cent of whom are women.
這家紡織廠有8千多職工，女職工佔百分之八十。
引導詞
即“引導定語從句的關係詞”
①引導非限定性定語從句的關係代詞：as,which,who
②引導非限定性定語從句的關係副詞主要有：when,where。
注意
不能用that作爲關係代詞的兩種情況：
①非限定性定語從句
②介詞+關係代詞
