"Los doctores principales están durmiendo."
Translation:The main doctors are sleeping.
"Principal" in English means "main" as well, so I don't know why "principal doctors" was not accepted.
Well, that's goofy. The unit is called "gerunds," but durmiendo isn't even used as a gerund, but as a present participle. Whatever!
I don't know why someone down-voted you. I was wondering the same thing.
It turns out, they consider the present progressive to be formed by estar + gerund even though I would consider sleeping to modify doctors and not are. Agree to disagree?
After all, it seems that Indiana University--Purdue University disagrees with spanishdict. These are links to more information on the gerund, but one says the gerund and present active participle are different; the other says they are different names for the same thing.
My answer "The principal doctors are asleep." was not accepted. Duolingo showed "The principle doctors are asleep." as a sample solution. I think someone got the word principal vs. principle mixed up here I reported it.
One "correct" answer DL gives includes principle to describe the doctors, when the correct word should be principal. Principle is a noun that means a rule or precept and is not equivalent to the adjective main or primary--so it is a mistake. If you get it, be sure to report it (use the report button) to get their attention. Gracias.