Swedish has the same system as English for this, and so does German I think, right? I remember school exercises about it, but it is not exactly complicated.So this one is trickier for Russians, I guess.

I don't think I make any grammatical mistakes in English anymore (??.... hmm.... ) but if I am tired I sometimes lose track of when to put an -s at the end of a verb.

I was in Germany last week actually, for work, and I was discouraged when I noticed that I still understand more German (which I never studied) than Russian.
I refuse to give up with Russian though, but Lord knows it's hard work. I really respect people who have managed to reach fluency.