"Our children are playing with the fish."
Translation:Unsere Kinder spielen mit dem Fisch.
In english, fish has two working plurals: fish and fishes. I thought this was using 'fish' in the plural sense, and translated as "Unsere Kinder spielen mit der Fische," which was marked incorrect, but if 'fish' was used in the plural sense it shouldn't it have been correct?
The sad truth is that you don't. You have to learn gender and plural with the word. For some type of words there are rules and for some there are hints:
but for the rest, you just have to memorize them.