"Do you have one?"
Translation:Haben Sie eines?
Because "ein" by itself doesn't mean anything; it is not correct grammar because it has not been declined accordingly. Here the sentence obliges you to decline in the accusative form ("do you have what? I have one"), meaning: NEUTER eins; MASC einen; FEM eine.
What's the difference between eins and eines? Eines looks plural to me, am I wrong?
"eins" is the numeral "one". (It is also used as a short term to refer to the time ("es ist halb eins" It's 12:30) or to the age ("er sprach schon mit eins" He could already talk at one year old)). "eines" is the declined form of "ein".
Amazing, thanks for the fast and clear reply! It clarifies your post above.
shouldnt all answers be correct in this question? If that isnt the case, what is wrong with the other answers?
Yes, theoretically, all answers, as long as they're DECLINED IN THE ACCUSATIVE FORM, are correct, but Duolingo has only put some of them as so, and the others grant an automatic and fabulous "wrong".
The second answer "Hast do jedes" actually means "do you all every/each (of them, of those, etc). "Jede" is a more precise word than "ein".
What would be the difference between "Hast du eins" and "Haben Sie einen"? Does the form for eins vary according to the nominative or does einen=eins?
They would be the exact same question - except the fact that the first is casual "you" and the second is formal "you" -, only declined differently, according to the context. This question "Do you have one?" requires that you decline in the accusative form (What do you have? -I have one.). Let me put it in context to make it clear: Ich habe ein Mädchen, hast du eines? (do you have one, a girl (neuter)?) OR: Ich habe einen Hund, haben Sie auch einen? (do you also have one, a dog (masculine)?). You can clearly see here, that even in the original question, you NEED to use the accusative form there, only because of the sentence type.