I'm still very slowly plugging away at French lessons in Duolingo and, although I have ultimately not progressed much over the last number of months, I'm at a point where I'm really learning that Duolingo, by virtue of the way lessons are structured, does a shit job of actually explaining the rules of grammar in general and verb conjugation in particular. I still find I'm often guessing about the proper conjugation because it's never really explained.
Honestly, as a native speaker I'm not always sure and I'm not the only one. There's always a confusion about some words and regional preferences.
For example:
Is it "une avion" or "un avion"
I always need to google it.