答：定语从句只能用关系代词which，而不能用that来引导的情况是：
1. 关系代词在介词后，用作介词宾语时。例如：
I'll never forget the day on which I first came to this school.
2. 引导非限制性定语从句时。例如：
Canada, which is North America, is the second largest country in the world.
3. 关系代词代表整个主句的意思时。例如：
My parents treat me like a a child, which I can't bear.
4. 先行词是代词that或those,或者是含有that/ those修饰时。例如：
Those flowers, which I saw there were very strange.
5. 定语从句距离指物的先行词较远时，或者说当定语从句对指物的先行词进行隔位修饰时。例如：
He sent me a gift of great value on my birthday which I liked very much.
6. 当先行词为代词that时，另避免重复，其后的定语从句的关系代词多用which( 附注：非正式英语中也可以用that). 例如：
She told me that which I had told you before.
7. 指物时，引导第二个定语从句的，习惯要用which.例如：
This is the building that was built last year and which is used as our lab now.