"Favorite" "bookmarked" or "flagged" rules.
It almost goes without saying that duolingo is impressive. Thanks! I do have a suggestion for a new feature, though.
Whenever a rule is explained, the user should have an option to mark that rule as needing more practice. The feature ideally would be automatic, the same way that duolingo already tracks weak vocabulary. But if that would be tough, then believe me a manual flag option would be helpful. You'd need a way to re-test certain rules, or even make sure that they come up again for future testing.
Who knows -- maybe you're doing that already! But while I found a link to practice weak vocab, I saw no such link for weak rules. Hence this suggestion.
Keep up the good work!