What role is "under vann" playing here, prepositional phrase or adjective? I wrote "...an underwater animal" and it was accepted, but that seems fundamentally different than "...an animal under water" (the main translation). To me, the first suggests that the animal is normally underwater (it has fins, gills, etc), and the second that the animal is currently underwater. At the zoo, for instance, you might see a polar bear swimming below the surface, but that doesn't make it an "underwater polar bear". Does this difference not exist in Norwegian?
In the case of the given sentence: En fisk er et dyr under vann. There are two indefinite nouns: fisk (en) and dyr (et). When an indefinite noun is introduced it is given with its indefinite article which agrees with the grammatical gender of the noun; masculine in the case of fisk and neuter with dyr. The indefinite article translates as a/an.
Hope this helps. Lykke til!
I'm guessing the Norwegian pattern is the same as the English. 'En fisk er et dyr under vann' = A fish is an animal under water. 'En fisk er et under vann dyr' = A fish is an underwater animal'
I also think the purpose of the sentence is to help us get an understanding of how to structure phrases that express location. The unfortunate thing in this example is that I immediately thought, 'That's not true! A turtle underwater is still a turtle'. Lykke til!