"You are girls, and we are boys."
Translation:Vy jste holky a my jsme kluci.
Check out this link, It helped me:
In reference to the discussion I had at https://www.duolingo.com/comment/24396423$from_email=comment&comment_id=24421287
Sorry, for not posting here initially - I didn't realize duolingo had this functionality - I'm learning, so bear with me.
Thanks for the explanation concerning why it must appear. In interest to anyone else who finds this page and doesn't want to read the whole discussion thread, the relevant part is
"... it may be helpful to quote from the really good Czech: An Essential Grammar by James Naughton (p. 76): The subject pronouns já, ty, on etc. are only needed for extra emphasis or contrast, or in combinations.
Já mám kufr, ale ona nemá. I have a suitcase, but she hasn't. Ty, já a Petr to uděláme spolu. You, I and Petr will do it together.
Mostly the verb is sufficient to indicate the subject, and you can simply omit the subject pronoun[...] Note that the author chose to treat the exception first, probably so the pro-drop message does not crowd out everything else. His kufr example is not as revealing as the sentence we are talking about here."
The case being that like tragram said, this is because of combination.
The link below has clarified this line a lot
Made me realize that dropping jsme is not wrong.
Made me realize that even though in some rare situation "jste dívky a my kluci" might be right. I shouldn't think of it as right until I understand how adding and removing the subject pronouns affects the message I am saying in the sentence.
You could still put jsem in the answer but I am glad duolingo didn't so we can be confused and ask why haha.