***** NOT A TEACHER *****
Good morning, Frank.
(1) You have asked a SUPER difficult question.
(2) I am sad to tell you that there is NO easy rule!
(3) I can only offer a little advice:
(a) Whenever you are confused about which word to use in a sentence, just post that sentence here, and one of the teachers or non-teachers will quickly tell you which word is correct.
(b) I am sorry to tell you that sometimes above/over and below/under have the same meanings.
(c) Maybe the most important thing to remember (maybe you already know this) is that "over" usually (NOT always) means DIRECTLY (in a vertical line) over your head: The roof of my house is over my head.
(i) "Above" is not necessarily DIRECTLY over your head. If you and some friends go outside, maybe you will say: Look at that airplane above us. Maybe the airplane is 10 miles away. It is not directly over your heads.
(d) "Under" also many times means DIRECTLY in a vertical line: The cat is hiding under my bed.
(i) Let's say that you and some friends climb and stand on top of a big mountain. Maybe you will say, "Hey, everyone, look at the beautiful valley below us." Maybe the valley is 10 miles away. It is not directly under your feet.
Have a nice day!
Student or Learner