What it is
The sister app of KotoSense, turned around: Japanese speakers learn Korean grammar. The interface and explanations are in Japanese; the sentences studied are Korean.
Where one Japanese expression — 「〜ている」, 「〜に」, 「〜てあげる／くれる」 — splits into several in Korean, the app shows the split in context and explains, in Japanese, how the meaning would change had you picked the other one. No pressure: no timers, no streaks, no rankings.
Where it stands
The study loop and the first content are being built. TOPIK is a search term, not a promise; the app claims no result and no official status. Release news goes on the blog.