As ridiculous as these 'nonsense' sentences are, I have to agree that I find them helpful in remembering the vocabulary. Also it forces you to think instead of relying on context, like one would be prone to do if the sentence were something like "The keys belong to the boy."
Well you see 1/3 dolphins will be sexually assaulted in their lifetimes. the other 2/3 are usually the assaulters. But anyways to combat this, its not uncommon in brazil for a dolphin to have keys to their dolphin apartment, most dolphins I personally know say that locking your doors and shutting your windows is the best way to prevent sexual crimes among dolphins.
Hello Libor! I think that duolingo tends to only accept the translations that are the closest to the original sentence. In the case of the sentence you suggested ("These are the dolphin's keys"), there is a closer translation:
"Estas são as chaves do golfinho".
I hope it helps! =)
Interesting, I tried "The keys are of the dolphin." which is the literal translation of the sentence we had, but it was marked wrong. They want "The keys belong to the dolphin." which would have been "As chaves pertinencem ao golfinho." or "The keys of the dolphin" which gets rid of the verb. Of course, I knew that my sentence was literal, but not the way we would say it in English. I am not sure that Libor's sentence is more different than the accepted versions. Wait, I think I have it! How about "The keys are the dolphin's." Let's just get rid of the preposition instead of the verb.