"Do you have some bread?"
Translation:Έχεις λίγο ψωμί;
Just to make sure I understand: there's another sentence that reads, "Εμείς διαβάζουμε λίγο από το βιβλίο" which means, "we read a little of the book." The reason why "από" is used in this instance is because the sentence is referring to a specific book. However, "Έχεις λίγο ψωμί;" is not referring to any specific piece of bread. You don't even know if the person has any bread, which is why you're asking in the first place. And that's why "από" is not used in that sentence. That's what I'm getting out of this, is that correct?
Of course both singular and plural forms are accepted. If your answer was rejected you might have had another error. You could send a screenshot and also report this on the exercise page under My answer should be accepted.
In addition you you should know that the plural form is used not only when referring to move than one person but also as a polite for one person, just as in French, German etc.
It's not correct because the original sentence is not inquiring whether "you have bread" but whether you have " some bread". That makes it a definite inquiry. We could just as well have asked..."Έχεις ψωμί΄" but we asked "Έχείς λίγο ψωμί," which is a very common and proper way to ask in Greek. There are are several other correct ways this could be translated but ..."Έχείς ψωμί;" is not one of them.
It would be understandable in english, as you said, but in greek and many other languages, the use of the 'definites' are different; while in english the definite article the is used to determine/specify something, which some times is omitted and gramatically correct, in greek it's a necessary part that will always follow a noun – you will see names with the article before them, for example (τ)η Μαρια. That's why the adverb λίγο is used in that sentence to express the correct idea. Correct me if I am wrong.