"Aren't you swimming with us?", I wrote. The software insisted that this was wrong, and offered, "Don't you swim with us?", which is how a foreigner might put the question, and thus instantly to be recognised as not a native English speaker.
it didn't accept my translation when i translated it as "you're not swimming with us" but why is it wrong? i thought present simple and continuous had the same form in Romanian
I wrote 'Are you not swimming with us' and it was marked wrong. Not sure it should have been but not confident enough to report it.
I would go ahead and report it. Duolingo normally accepts the present continuous as a correct translation and should here as well. Same for the contracted form "Aren't you swimming with us?" which would be the most natural English phrasing.