"Few airplanes fly above."
Translation:Fent kevés repülőgép repül.
Is there any real difference between fent vs. fölött vs. felett? Is there even one?
fent stands on its own; fölött/felett are postpositions that go with a noun. So "above the tree" would be a fa fölött, for example; you can't say a fa fent; nor can you (I think) say fölött without a noun in front of it.
English has quite a few words that can be either adverb or preposition, which Hungarian separates.
But compare e.g. "next to", which is only a preposition - you can say "He is standing next to the house" but not "He is standing next to" (it would have to be something like "He is standing at the side").
As for fölött and felett, I believe they're completely interchangeable.
In the previous chapters, repülő was introduced as a synonym for repülőgép. However, this particular exercise doesn't accept repülő. It's a bit inconsistent. Is repülő a colloquialism for repülőgép or is it not?
It seems to be an abbreviation but by itself it could also just mean "a flyer" such as a toy that flies whereas with gép at the end it is definitely a flying machine, ergo, an airplane. :)
I'm not sure I understand why, "Keves repulogep repel folott." is not accepted.
As mizinamo pointed out above, fölött is not a stand-alone word, but rather a postposition that needs to follow a noun (phrase). When "above" is to be used as an independent adverb, it should be translated as "fent". Also, you have a typo on "repül".