Fisch is a masculine noun, and in this sentence it is in the accusative case:
If you need more info on cases:
So is this the case with ALL masculine nouns when they are in the accusative case, or just certain words? I have been struggling so much with ein/einen/eine and I thought i had figured out that "den" words were also referred to as einen, but this has thrown me off quite a bit. All this nominative accusative stuff is so complicated, I am trying to understand but maybe I can get a few examples?
When does "essen" mean "are eating" and when does "essen" mean "eat" ?
German does not make this distinction, so if there is no context that determines whether an action is habitual or repeated ("they eat") or is happening now ("they are eating"), both translations could be valid.
In general, both translations (present simple and present continuous) will be accepted if the German sentence has present tense.
If you have a question about a sentence that was not accepted, please always quote your entire answer.
Even better, take a screenshot that shows what you typed (which is sometimes not what you wanted to type, if you made a mistake), upload it to a website somewhere, and add the URL to your comment.
If this is being translated to American English...nobody here in America says "we are eating a fish." Nobody. Ever. We say "we are eating fish." Duo should absolutely accept that answer. So much so, the literal translation, "we are eating a fish" should be considered incorrect and marked wrong.
"Why is we eat fish not" correct?
I'm assuming that your quotation marks are placed incorrectly and you wanted to ask Why is "we eat fish" not correct?.
If so, then it's because the German sentence talks about eating einen Fisch "a fish / one fish" -- that is, one whole fish. Whereas "we eat fish" in English refers to any quantity of meat that comes from one or more fish: it could be a little bit of salmon or two entire kippers, for example.
Thus "fish" is not a good translation for einen Fisch.
And if your quotation marks were deliberate: Why is we eat fish not is not a correct sentence in English. Thus it can't possibly be a correct translation for Wir essen einen Fisch.