Trainers
Spaced-repetition practice.
Questions you get wrong come back in review rounds of up to 20. A missed question waits 8 hours at first. Each further miss in a row doubles the wait, up to a week. It stays in the rounds until you answer it right twice in a row.
In a chapter you can also switch to its hardest quarter, the questions learners miss most, or to only your mistakes that are due for review.