"They are not a couple."
Translation:Oni nie są parą.
Why is "para" not in the genitive? I thought you used the genitive when negating?
The construction here (a copular sentence) is "... jest/są + instrumental", for example "He is a worker = On jest pracownikiem (instrumental of pracownik)". Here, parą is the instrumental of para. Only accusative (direct object) changes to genitive when negated, other cases don't do that.
Parą is similar to "pareja" in Spanish. Another word more easy to remember. Great. :)