It is the same answer as for your other post: https://www.italki.com/discussion/238689
and like John said, it depends on what you are trying to say, as you can use both.
The sun is above the mountains = directly overhead the mountains, in the sky above the mountains.
The sun is over the mountains = over the mountains, higher up in our vision that the mountains, but can also mean a little beyond …. but refer to the other post. For 'sun' you could use both/either, but for a cloud, it could also mean 'beyond'.
Read more English books. I can't explain it to you better, nor in Turkish.