The goal: fill the bar AND earn 3 stars. To master a word you need two things: its progress bar nearly full (from practicing the 4 modes) and all 3 stars ★ ★ ★ earned (one per day, in reviews). See both in the Word List.
To earn a day's star: in a Review session, answer the word in 2 different modes, score 60% or higher overall, and get nothing wrong that day. A star is a clean day — partials and hints still count, but a single wrong answer means no star (the word comes back tomorrow to try again). A review gives you the word in 2 different modes automatically.
How can it be 60% and nothing wrong? Answers aren't just right/wrong — each earns a score: fully correct = 100%, a partial answer or one using a hint = about 15–75%, and a wrong answer = 0% (which breaks the clean day). So "nothing wrong" means no zeros — but partials/hints can still pull your average below 100%. That average just needs to reach 60%.
Only reviews count. Practicing modes on your own builds the progress bar and helps you learn, but it doesn't earn a star — reviews are the honest test (limited attempts, 2 modes), so mastery can't be faked.
The bar rises once a day. Each mode's share of the bar climbs once per day you get it right — ten right answers in one sitting count the same as one, because lasting memory is built by returning on different days. A wrong answer can lower it any time.
Reviews space out. Get a word right and it comes back after 1 day, then 3, 7, 14, 30, 60. Miss it and it returns the next day for extra practice. The app schedules everything — just clear what's due.