非限制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，可用来说明前面整个句子的情况或主句的某一部分。如：

She heard a terrible noise，which brought her heart into her mouth．

David studies hard and is ready to help others，which is what his parents expect．

2. 在非限制性定语从句中，不能用that，而用who, whom代表人，用which或as代表事物。关系代词作定语时用whose。如：

1）He loved his parents deeply, both of whom are very kind to him.

2）In the basket there are quite many apples, which have gone bad.

3）There are forty students in our class in all, most of whom are from big cities.

4）His mother, who loves him very much, is strict with him.

5）China, which was founded in 1949, is becoming more and more powerful.