定语从句中，先行词为事物，只能用which而不能用that的情况如下：

1、引导先行词为事物的非限制性定语从句时。例如：

My dog ,which is now very old, became ill yesterday.

我家的狗昨天病了，它已经非常老了。

The pen, which writes very well, cost me RMB 20.

这支笔花了我20元，它非常好写。

2、关系代词前有介词时。例如：

This is the classroom in which we studied last year.

这是我们去年在里面学习的教室。

Have you ever been to a farm on which there is a small zoo?

你曾经到过有个小动物园的农场吗？