1. 非限制性定语从句常用来补充说明其修饰的先行词，对所谈及的人或事物提供进一步的说明或解释，如果将它去掉，句子的意义仍然是完整的；书写时，常用逗号将其与句子的其他成分分隔；译成汉语时，常把它当作一个独立的句子来翻译。如：
This novel, which I have read three times, is very touching. 这本小说很感人，我已经读了三遍。
After graduation, she decided to stay in Shanghai, where she spent her childhood and four years of college life. 毕业后，她决定留在上海，在那里她度过了她的童年和四年的大学生活。
2. 当先行词是专有名词或具有“唯一性意义”的名词（如her mother, my father等）、人称代词（如you, they等）或有指示代词修饰，其后面的定语从句大多是非限制性的，故书写时常用逗号分隔。如：
John was eager to go to the hospital to see his daughter, whom he loved very much. 约翰急于想去医院看望他的女儿，他非常爱她。
Alice, who is one of my best friends, went to America last year. 爱丽丝去年去美国了，她是我最好的朋友之一。
3. 非限制性定语从句修饰整个句子时，引导词一般用which或as，两者有时可以换用；如果引导词作主语，从句的谓语动词要用第三人称单数形式。如：
She is patient this time, as she has never been. 她这次很耐心，过去可从不这样。
Yellow Stone National Park, which is in Wyoming, is one of the most beautiful national parks in the world. 黄石国家公园位于怀俄明州，是世界上最美丽的国家公园之一。
4. which 引导的非限制性定语从句必须放在主句之后，不能放在主句之前；as 引导的非限制性定语从句则既可放在主句之前，也可放在主句之后。如：
He lost his way, which delayed him considerably. 他迷了路，这使他耽搁了很久。
The journey was not nice, as we had expected. / As we had expected, the journey was not nice. 正如我们所预想的那样，这次旅途并不愉快。
5. 非限制性定语从句为“引导词（作主语） ＋ 及物动词 ＋ 宾语”结构，或引导词在非限制性定语从句中作宾语并带有宾语补足语时，一般用which 作引导词。如：
David was late again, which made his teacher very angry. 大卫又迟到了，这使得他的老师很生气。
He wants to be a scientist, which I find impossible. 他想当一名科学家，我觉得不太可能。
6. 非限制性定语从句为否定式或含有否定意义时，常用 which 作引导词。如：
Harry said he had been to China, which was not true. 哈里说他去过中国，那不是真的。
The boy thinks the moon is bigger than the earth, which is not right. 这个男孩认为月球比地球大，这是不对的。
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