Keigo is Japanese honorific speech. This drill gives you a plain verb and asks for its respectful form (尊敬語), its humble form (謙譲語) or its polite form (丁寧語) — 行く becomes いらっしゃる, 参る or 行きます depending on who is speaking about whom.
Every item is hand-verified, and each answer is explained, so you learn the pattern rather than a list.
Every card has a temperature. Get it right and its chance of coming back halves; miss it and it doubles. K2 picks the next card at random, weighted by that chance, so a session keeps returning to whatever you are getting wrong and lets what you know fade into the background.
When the cards you are working on have cooled, a new batch joins them, so you always study at the edge of what you know. There are no review dates and no daily queue to fall behind on. Every drill is free and works without an account, and everyone gets 20 free AI uses a day; past them, signed-in AI practice runs on your Bike4Mind credits.