that引导的宾语从句作介词的宾语时,that不可省略那为什么会有介词后不用that用which?

请举例要不然不好回答.是这样的吗比如yesterday I informed my students of that our classroom is on the 3rd floor.或者yesterday I informed my students of which floor our classroom is on
再问： 是的为什么第一句后面可以用that，不是介词后面不能有that吗？
再答： 我估计你是把宾语从句和定于从句弄混了。宾语从句可以用that。定语从句介词后不用that，要用which。