非限制性定语从句的概念：
非限制性定语是对被修饰名词或代词的附加说明，它不是必需的，如果去掉，也不会影响句子的意思，它与被修饰名词之间通常用逗号分开。
如：The travellers, knowing about the floods, took another road. 游客们知道发了大水，都改道走了。
The boys, wanting to play football, were disappointed when it rained. 那些男孩子想踢足球，因为下雨感到失望。
非限制性定语从句用法：
1、引导非限定性定语从句时，只能用which(不用that)。
例如：Heat is another form of energy, which is as important as other kinds of energy.
热是另一种形式的能量，与其他形式的能量一样重要。 （从句表补充说明，而且关系代词which不能换成that。）
2、引导非限定性定语从句的which可以指代前面的先行词,也可以指前面整个句子的含义。
例如：That Peter will marry Alice, which has not been announced yet, has spread around.
彼特要娶爱丽斯这件事还没宣布，却已传得沸沸扬扬。（句子中的which指“彼特要娶爱丽斯”这整个句子的意思。）
3、除which外，还可用when，where，who等关系代、副词引导非限定性定语从句。
例如：After graduation, I decided to stay in Chongqing, where I spent my childhood and four years of college life.
毕业后，我决定留在重庆，在那里我曾度过了我的童年和四年大学生活。
Albert Einstein left Germany for the United States during World WarII, when Jews were badly treated in Germany.
第二次世界大战期间，爱因斯坦离开德国去了美国，那时犹太人在德国受到不好的对待。
4、在限定性定语从句中作宾语时，引导词可以省略，但引导非限定性定语从句的关联词不能省。
如：He was eager to go to the hospital to see his stepmother, whom he loved and respected as his own mother.
他急于想去医院看望他的继母，他把他的继母当作亲生母亲一样热爱和尊敬。
The American journalist(whom/who) the announcer mentioned in the news broadcast is said to have been killed by the gangsters.
播音员在新闻广播中提到的那位美国记者据说已经被匪徒杀害了。
两例中的关系代词都在从句中作宾语。由于第二例是限定性定语从句，可以省略关系代词；第一例中的引导词不能省略，因为它引导的是非限定性定语从句。
5、表示“正如”的含义时，通常用as引导非限定性定语从句，也可用which引导；但置于句首时，只能用as引导。
如：China has basically succeeded in defeating SARS, which/as we have expected.
正如我们所预料的那样，中国已基本上战胜了“非典”。
As is well known to everybody, Tai wan is an inseparable part of China.
众所周知，台湾是中国不可分割的一部分。
但是当非限定性定语从句是否定含义时，就只能用which(而不用as)引导。
如：He didn't win the championship, which I hadn't expected.
他没获得冠军，这一点是我没预料到的。
非限制性定语丛句中as, which的区别：
1、which引导非限制性定语丛句代表前面的整个句子的时候，一般是对主句的结果的说明。
如： He grows too fast, which makes him taller than his classmates.
2、as引导非限制性丛句代表前面整个句子时一般来讲丛句的谓语动词有三种：
A. 含有be动词：
如：He failed the exam, as is natural.
B. 实意动词的被动形式：
如：As is reported, the fire caused a great loss.
C.感官动词和意识类动词如：
如：see, hear, notice, know, learn, realize 等。
As you know, I am a teacher.
3、as可翻译为正如，它引导的丛句可位于主句之前，也可位于主句之后；which引导的该丛句只能位于主句之后。
例1：__A___he realized, I was very useful to him.
例2：This elephant is like a snake, ___A__anybody can see.
例3：The sun gives us light and heat, __B___makes the plan tgrow well.
A. As(as)
B. which
C. that
D. who
限定性定语从句与非限定性定语从句的区别：
非限制性定语从句的关系词：
非限定性定语从句的使用规则及注意事项：
1、which引导的非限定性定语从句是用来说明前面整个句子的情况或主句的某一部分。
2、在引导限定性定语从句时，that有时相当于in which, at which, for which或at which。其中，介词的选用，依据从句中的动词所需搭配的介词来选用。例句：
① Attitudes towards day dreaming are changing in much the same way that(inwhich)attitudes towards night dreaming have changed.
人们对白日做梦的态度正在改变，这与人们对夜间做梦的看法的变化有非常相似之处。
② I like the music for the very reason that(for which) he dislike it.
我出于某种原因喜欢这种音乐，而他恰恰与我相反。
③ We arrived the day that(on which) they left.
刚好我们到的那天他们走了。
3、as有时也可用作关系代词。
4、在非限定性定语从句中，关系词不能用that。