which引导定语从句时,既可引导限制性定语从句,也可引导非限制性定语从句.
引导限制性定语从句时,which前加逗号,如 A shop should keep a stock of those goods which sell best. 此时which指代它前面的一个词 goods
引导非限定从时,which前要加逗号,既可指代前面的一部分,如 When deep in thought, which he often was, he would forget all around him.
也可指代前面的一整句话,如 Internet is so interesting, which makes all possible happen.
这里that 如果直接改成which是可以的,前面加逗号就不行了,因为这里which指代的是past这个词,也就是说which在这里只能引导限制性定语从句.