"У мене є м'яч."
Translation:I have a ball.
In this case, it is rather some indefinite ball:
- У кого є м’яч? (Who has a ball?)
- У мене є м’яч. (I have a ball.)
In this case, some particular ball is meant and it should be known from the context which one:
- У кого м’яч? (Who has the ball?)
- У мене м’яч. (I have the ball.)
I think in this case if there is a distinction its a rather fine one - and not really useful to teach at an early stage (what i think this is designed for, or most appropriate for) so best either to accept both (as more or less correct: perhaps. "you are almost right") or simply not use this example (until a later stage when its more useful for people to learn the finer points)