非限定性定语从句是英语语法中从句的一种，是定语从句的分支之一。作用是对所修饰的成分作进一步说明，通常和主句间用逗号隔开，将从句拿掉后其他部分仍可成立。运用非限定性定语从句，尤其要留意其语法运用与其所表示的东西。
1 who引导的非限制性定语从句
  Our guide，who was a French Canadian，was an excellent cook.
  我们的向导，一个法裔加拿大人，擅长于烹调。
  My gardener，who is very pessimistic，says that there will be no apples this year.
  我家的园丁特别悲观，他说今年将不结苹果。
2 whom引导的非限制性定语从句。
  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，介词可位于句末。
  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.
  史密斯先生是一位闻名的科学家，我从他那儿学了很多东西。
3 whose引导的非限制性定语从句。 
  whose是关系代词who的全部格形式，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.
  那位小男孩学习很努力，他的父亲是位工程师。
  Above the trees are the mountains, whose magnificence the river faithfully reflects on the surface.
  在树林的高处是山，其壮美的景完全映照在河面上。
  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。
4 which引导的非限制性定语从句。
  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。
  ① which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与表示职业、品行等的名词。
  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.
  水是一种澄澈的液体，有很多用途。
  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，也真是如此。
  ② which指代主句中的形容词。
  如：She was very patient towards the children，which her husband seldom was.
  她对孩子们很耐性，她丈夫却很少这样。
  She is always careless，which we should not be.
  她总是马虎大意，我们可不应当这样。
  ③ which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。
  ④ which指代整个主句。
  如：In the presence of so many people he was little tense, which was understandable.
  在则多人面前他有点惊慌，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，假如是这样，他就得动手术。
  When deeply absorbed in work，which he often was，he would forget all about eating and sleeping.
  他常常全神贯注地工作，这时他会废寝忘食。
5 when引导的非限制性定语从句。
  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。
5.1 where引导的非限制性定语从句关系。
  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。
6 as引导的非限制性定语从句。
  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、表语或宾语，且引出的从句位置比较敏捷，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·吐温是一位宏大的美国作家。（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，他过去常常这样。（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.
  两兄弟对此确定都满足，这项确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，台湾是中国不行分割的一部分。(as在从句中作宾语)
  “介词+关系代词”引导的非限制性定语从句在介词后引导非限制性定语从句。
  难：句意相同
  as it is known to all () (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 (主语从句，it是形式主语)
  既：As is widely known, the moon is closer to the us than the sun.(定从)= Thatthe moon is closer to the us than the sun is widely known.(主从)= it is widely known thatthe moon is closer to the us than the sun.(主从)