We should be encouraged to practice the difficult part, not the trivially easy parts. But as of March 2022, repetition in Duolingo is unbalanced in this regard.
Yes, I am riffing on the metaphorical use of “unbalanced” to refer to people who are mentally unstable, although I don’t think this literally applies to the good folks at Duolingo, and hopefully won’t be the effect on users either. Although it certainly doesn’t help.
You see, Duolingo has a lovable feature now that lets you “repair” bubbles (themes) that you have already maxed, either by getting them to “legendary” (blue) or just to the maximum level, 5. When some time has passed, the bubbles appear broken with cracks through them, and you are offered double XP for repairing them. This is a pretty good motivation, since XP is what you need to maintain your daily streak. (Although in my case I have only picked 20 XP per day, so it doesn’t really matter. Still, it gives a sense of accomplishment and makes the process of learning more like a game.)
So far, so good. The problem is that the system for picking bubbles to repair pays no attention to whether those lessons were easy or hard. It makes no sense to repeat “Basics 1” in the Finnish course, or at least not more than once a year, since it teaches things you use all the time later on. Likewise, you wouldn’t want to repeat Hiragana 1 in the Japanese course unless you had been away for a year, because you use Hiragana in almost every lesson for the rest of the course.
Duolingo starts very gently, something I deeply appreciate. But after a while, the learning curve starts rising steeply, especially for languages that are very different from English. This is because as you progress, Duolingo adds not only new vocabulary but also new grammar features at the same time. And on top of that, it increases the length and complexity of sentences. And on top of the top, the spoken examples speed up. I assume this will become a problem at a different time for different people, depending on your intelligence and your background in similar languages.
For me in Finnish, the steep uphill started in earnest around the middle of Unit 2, with the lesson called Coffee. This introduces a couple of grammatical features we don’t have in English, like the two different meanings of “to drink”. (“I want to drink something” versus “I want something to drink”. An Englishman will normally say these mean the same thing, but that is not so in Finnish. “Something to drink” is basically translated the same way as “something drinkable”. So one sentence focuses on the nature of the action, but the other sentence focuses on the nature of the object, and the grammar is different.)
Of the next two lessons, Europe was fairly easy because it concentrates on the new grammatical case, inessive. In Finnish, you add -ssa/ssä at the end of the word instead of the preposition “in” that we use in English (and similar prepositions in other Germanic and Romance languages). So that is odd for us, and Duolingo properly devotes the whole lesson to this, teaching only a few new words, and then mainly names of European cities. This is the best way to do things, I think, concentrating each lesson either on new vocabulary or a new grammatical feature, not several things all at once.
My point (finally!) is that I need to repeat those difficult lessons very soon after I have learned them, and then with gradually longer intervals for a while. But the super-easy lessons don’t need to be repeated, or only every half year perhaps. It makes no sense to get the same XP for repeating the words for boy and girl as I get for repeating obscure Uralic grammar. If available at all, the first should count for 5 XP and the latter for 50 XP. That would make sense. And then move those numbers down the language tree as we progress, so that the newest lessons would be repeated more often and give more XP. This would also fit what we know in the 21st century about the neurobiology of learning.