"The three brothers were swimming in the pool."
Translation:Los tres hermanos estuvieron nadando en la piscina.
The unit is Verbs:-Past-Imperfect, and nadaban is the imperfect tense. Furthermore the unit includes plenty of examples of both 'imperfect tense' and 'estar + participle'. It's just extremely inconsisent about which form it will accept.
And yes, I've reported several mistakes, some days ago and they still haven't been fixed. I was beginning to wonder if I was wrong all along?
People argue about what form to use (see below). I have been taught, in books, Argentina, and Mexico, that the imperfect describes a setting (nadaban) in general and the progressive is used only to emphasize that something is/was happening at a specific moment. If I wanted top say "Three boys were swimming when lightning struck" I'd use "nadaban" and the preterite for the lightning. If were testifying in court and was asked, "What, exactly, were the boys doing when the lightning struck?" I'd use the progressive. Inviolate rule? I doubt it.
When I have raised an issue with Duo, they usually tell me why they won't fix or tell me they have fixed it, but it sometimes tales a loooooong time to get a response.