Oh, I must have thought this was the Plurals skill. From the Tips & Notes for Plurals:
-а/-я -nouns end in ы/и
-ь -nouns, feminine end in и
most consonant-ending masculines end in ы/и
-о/-е -nouns end in а/я
some consonant-ending masculines end in а/я
And then there are the irregular plurals, which I'm not very familiar with. Hope this helps! :-)
You have to understand, товарищ, that this comment was written one year ago. A lot has changed over that past year. Whether there are any Tips Notes left on the Russian course or not, I do not know. If you say that there are no Tips Notes left, and you have dubble checked below the skill, I believe you. However, it is not my fault they have been removed, nor it is "A_User"'s fault.
If you feel the need to go around complaning about Duolingo, please do not do this in a random Russian thread. Go to the public discussions. If you still believe that Duolingo is a "garbage site", you are free to leave at any moment. No one's forcing you to learn Russian. Just don't disturb all of us who are actually taking language learning seriously.
Duolingo member since 2015.
Владеет русским языком.