"Jenny is not a girl but my cat."
Translation:Дженни — это не девушка, а моя кошка.
Wouldn't using the genitive here be like saying Jenny isn't any girls, instead of sayi g she isn't a girl? Normally when I see the genitive used with a negative it's saying there isn't any of something.
Kundoo's response is not an answer.
Even so, with нет you'd have to use the genitive девочки.
Девочка refers usually to a young girl, like a child. Девушка is more like a young woman or can also refer to a girlfriend.
Но is used when there is a meaning of "despite". Учитель зашёл в класс, но дети разговаривают - The teacher entered the classroom, but kids are talking. Он не учил песню, но знает слова - He didn't learn the song, but he knows the lyrics.
Well, "Jenny" is a female name, so one would assume we are talking about a female cat - "кошка". Not that it's impossible to name your male cat ("кот") Jenny, but I don't blame the creators of the course for not anticipating this answer.
of course you're right :-) sometimes people have strange ideas like naming a cat "собака" , one of my friend did it :-D
Just found this and now I understand: https://www.duolingo.com/comment/11536858/A-Guide-to-Using-%D0%AD%D0%A2%D0%9E