非限定性定语从句：非限定性定语从句的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
1. which引导的非限定性定语从句来说明前面整个句子的情况或主句的某一部分 ，that不能用于引导非限制性定语从句
例如:
She asked for his help which he gladly gave. 她要求他帮忙,他愉快地帮助了她。
It was a century during which the country suffered continuously from wars. 一个世纪过去了,在这期间这个国家不断遭受战乱之苦。
Attitudes towards daydreaming are changing in much the same way that(in which) 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。
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。
We arrived the day that(on which) they left. 刚好我们到的那天他们走了。
3. 有时as也可用作关系代词
4. 在非限定性定语从句中，用who, whom代表人，用which代表事物.
5.当先行词是专有名词或物主代词和指示代词所修饰时，其后的定语从句通常是非限制性的。
例如：
Charles Smith, who was my former teacher, retired last year. 查理·史密斯去年退休了，他曾经是我的老师。
My house, which I bought last year, has got a lovely garden. 我去年买的的那幢房子带着个漂亮的花园。
This novel, which I have read three times, is very touching. 这本小说很动人，我已经读了三遍。
6.非限制性定语从句还能将整个主句作为先行词, 对其进行修饰, 这时从句谓语动词要用第三人称单数，
例如：
He seems not to have grasped what I meant, which greatly upsets me. 他似乎没抓住我的意思，这使我心烦。
Liquid water changes to vapor, which is called evaporation. 液态水变为蒸汽，这就叫做蒸发。
参考资料：http://baike.baidu.com/view/576409.htm