over/above/ on 都可以表示“在.上之意.具体区别：
over：长表示恰好垂直的位置关系,或者含有空间上接近的含义.其反义词为under.如：
There is a bridge over the river.
My pen is now under your desk.
above:只笼统的表明“高于.;在.之上”,多指在相对较高位置.其反义词是below.如：
We flew above the clounds.
表示“高于某个抽象水平”或“温度zero degree以上多少度”用above.
His work is well above the average.
The temperature is three degrees above zero.
【The temperature is above/over 30 "C.温度高于...时可互用.】
on:"...表面上”,强调与表面接触.其反义词为beneath[正式语体,现代英语中多用比喻义.如：
There are some books on the desk.
The village was situated beneath a wooden hill.村庄位于林山下.
As a scholar,he is far beneath his brother.
