In one of the generic Spaced Repetition Learning programs I used earlier – either Supermemo or Mnemosyne – you could mark a card as hard even if you remembered it correctly. I would like to have this as an option in Duolingo as well. I believe that in other programs, you were expected to grade your answers from 3 difficulty levels, or perhaps it was 5? In Duolingo, I would be satisfied if there was just a small button off to the side with some symbol on it. The dumbbells are already used for the training section, so perhaps a weight or some such, or a “steep angle” traffic sign? Anyway, it should be an option to press this to signal that you would like to have this exercise repeated earlier than the rest.
The button should not be active if the on-screen exercise contains a new word or your first meeting with a new grammatical feature. These are prioritized automatically, I believe, so pressing the button might give the usual feedback (perhaps a small sound) but not be counted. But if you get an exercise with only familiar words and you still struggle and only get it right because you lucked out to pick the correct answer out of the two or three you were thinking of, then this button would be very useful. It would allow you to repeat the exercise during training sessions without losing a heart.
Today, the only way to mark a sentence for repetition is to deliberately get it wrong. For us paying customers who have unlimited hearts, this is a viable strategy (since we have unlimited hearts), but it just feels wrong so I pretty much never use it.
Hopefully, Duolingo notices whenever we use the hints feature (which I love, by the way). But sometimes there are no hints, but the exercise can still be so hard that I only got it right by sheer luck. And there is no way I can tell Duolingo that. But there ought to be. We should not need to keep a text file where we write down the hard phrases and practice them manually. It breaks the immersion and the playfulness of the learning game.