Is "Peter gyakran repülőgépeket hall" a valid word order here? I know it says not to report mistakes here, but I don't know if this is something that should have been accepted or something I don't understand fully...
Yes It's good.
How about Péter repülögépek gyakran hall?
'Repülőgépek' is only the nominative form but the verb 'hall' requires accusative. It means that 'repülőgépeket' have to be used: 'Péter repülőgépeket gyakran hall.'
What about Péter repülőgépeket gyakran hall? I think this is right, so I've reported it but if it's not if like to know why.