"You sit under the tree."
Translation:A fa alatt ülsz.
Because we do not see what you wrote. Did you also follow the rules of Hungarian grammar? Do you mind sharing what you wrote?
"Fát" would be in the accusative. That is used only when the object is the direct object of the sentence.
- Látom a fát. - I see the tree.
- Szeretem a fát. - I like the tree.
- Ültetek egy fát. - I plant a tree.
But here we have "under the tree". In Hungarian, that is a noun in the nominative plus a postposition:
"a fa alatt".