the用法

http://eason982.blogspot.com/2008/11/blog-post.html

on top of 是在某個東西的上方處
ex. 放在書櫃上面沒格的雞毛撢子

on the top of 是某個東西的上部
ex. 放在書櫃最上面那層格子的書

in front of 在某物前方
ex. A padestrian is standing in front of the bus.

in the front of 在某物的前部裡
ex. A bus driver sits in the front of a bus.