I really like the way duolingo works, but it could be a lot better if you could mark words / sentences / expressions you would like to repeat more often because you have difficulties remembering them. I found that the same words are repeated over and over, although I never had difficulties or at least got to know them by heart. Other words or expressions are rarely repeated in the training sessions.
Example: in the French course I often see "la femme", "une femme", "des femmes", "un enfant", "les enfants" and other very simple words. I can't remember making a mistake, and yet they turn up disproportionally often. On the other hand, words I have difficulties with like "appartient", "améliore", "doute", "oiseau", "fourmi" and others are hardly ever asked.
I don't know how the system decides which words to repeat, but it would be great if you could mark words you want to see more often - and maybe also mark words you don't want to see as often. I was shown "la femme - ?", "die Frau - ?", "Frauen - ?", "l'enfant - ?", "les enfants - ?" and variations of these words 75% of the training session, although I never had a problem. I feel that I'm wasting time that I could and would like to use much better by focusing on problem areas.
By the way, this is not just a problem in the French course, but also in the Spanish course. I assume it's not different in other courses.
You are completely right. I had this feeling as well, but try to see it from another perspective: your brain loves familiarity. So when a radio station wants to establish a new song they will play a famous red hot chili peppers song right after the new tune. You heard the song a lot, you like it and so your brain connects the following or preceding new song a lot better in a same way. You are more likely to like and remember the new tune. Same with Duolingo words…
Sometimes I am surprised by my own brain, when I just write down a word that is in my head, even if I feel like it’s wrong or the spelling is wrong. But the connection is there, and builds over time. You can’t decide by yourself which words you know, but you can train single words on Duolingo. click on words (or https://www.duolingo.com/words) and you can see all your word strength and weaknesses. Try the app ‘tinycards’ as well – you can build your own flashcards and train words you still are not so confident with.
In the End you have to trust the algorithm, I guess. By that, you will also improve better, than if you are uncertain that it really works