It seems like "where is a boy?" has to be the correct translation, but that wouldn't make sense. I wrote "Where is the boy?", and it was accepted, but I don't understand why just "chłopiec", by itself, can correctly translate to THE boy.
Polish doesn't have any articles. When you translate to English you need to add them where appropriate.
The phrases on DL are taken out of any context... "Where is the boy" would be correct, if in previous phrase it was specified what a boy we are talking about, so the phrase in itself should also be correct.
I think it's this way because in English, "where" is nearly always talking about a specific or previously mentioned boy. "Where is a boy" would occur in fewer circumstances.
To put it simply, nominative case is used here, because „chłopiec” is the subject of the sentence, not the object of the verb „jest”.
If you wrote „Gdzie jest chłopcem” it would mean that something called „Gdzie” is a boy.