I would like to visit the place____ my brother mentioned in his last letter. 这里填的是where 还是that 为什么? 

This is the school___I studied English two years ago.这里填 where 还是which

Some students copied from small pieces of paper on ___they had prepared for the exam.  这里说是因为介词提前填which ,那本来这个on 是从哪里来?

球解答!怎么去区分这些that 还是where 还是which?