首先关系代词which在定语从句指物，在定语从句中作主语、宾语或定语，既可引导限制性定语从句，又可引导非限制性定语从句。
A、引导限制性定语从句
E.g: A shop should keep a stock of those goods which sell best. 商店应存有最畅销的货物。
B、引导非限制性定语从句。 （ 1 ）用来指代一个句子。
E.g:Internet is so interesting, which makes all possible happen. 因特网是如此有趣，它使任何事都有可能发生。
2、用来指代句子的一部分。
E.g:When deep in thought, which he often was, he would forget all around him.
他常常陷入沉思，这时他就会忘掉周围的一切。
（ 3 ）如果要引导两个非限制性定语从句，第二个 which 前要加 and.
E.g:He bought a book, which was written by Lu Xun, and which he decided to give to his friend.
他买了本鲁迅写的书，他决定送给朋友。