This is a good rule. Just remember that it doesn't always work. For example ett foto (photo) is ett foto, fotot, foton, fotona. I think it would really helpbif Duolingo showed the groupings of both verb and noun conjugations. Apart from irregular verbs ndbthe like, they are pretty standard
I keep making the same mistake. I know why I'm making it, but its frustrating. I spent so long learning German that 'en' at the end of a word just looks like its the plural. And 'är' looks and sounds like 'are' so I keep translating "the x is y" as "X's are y" and EVERY TIME I do it I facepalm at my own stupidity.