I agree with phuvtuo. They are teaching us sentences based on words learned, not trying to make the most natural sounding English. And just for the record, I said this exact thing after building a bookshelf. I made one shelf shorter than the others for specific short books I realized were in storage, not in my house.
You are missing the point of what we are doing in these translations. We are matching the grammar of the original language to show we understand its grammar. We have to adhere to forming correct English sentences only. We are not supposed to be changing the grammar in order to make the most natural sounding English sentence. The English sentence is not incorrect. There are times when somebody including me would say it. It doesn't violate any rules of English grammar. It is a correct sentence that matches the grammar and vocabulary of the Arabic sentence which is the entire point here. The Arabic sentence is not in the plural so the English sentence we give is not in the plural.
Imagine the chaos if everybody gave their own best sounding translations or how they say it while totally ignoring the actual grammar of the Arabic sentence. The course contributors would have to write an unreasonable number of accepted sentences to accommodate this.
Just translate the sentences as closely as you can to match syntax and vocabulary instead of trying to win some award for most poetic or natural sounding English sentence.
There is a sign for it, but some reason behind my humble understanding, Duolingo does not put it down.
The marker for -un is:
or sometimes (not common now i guess):
Following suit, the same with -an and -in Nunations:
Notice that the -an Nunation is the only one that requires adding Alif to the word's end (long story short, spoken language decrees that). Only in few instances (e.g. ending with Ta Marbúta ة) where this Alif is not added for -an Nunation.