定语从句如果修饰“物”,用关系代词that的时候较多,有时也用which.例如：
It's a computer which / that costs six thousand yuan. 这是一台价值六千元的电脑.
当这个代词在从句中用作宾语时,往往都会把它省略.例如：
The present(that / which) you gave me is very beautiful. 你给我的那件礼物非常漂亮.
在介词的后面只能用which.例如：
This is the book about which they are talking. 这就是他们正在谈论的那本书