Spaced repetition, explained: from SM-2 to FSRS
Your memory of a word decays on a predictable curve. Review it at the right moment — just before you would have forgotten — and the curve flattens, buying you weeks, then months, then years. Review too early and you waste time; too late and you start over.
Classic algorithms like SM-2 approximated that timing with fixed rules. FSRS, the Free Spaced Repetition Scheduler, models forgetting directly, adapting to how you actually remember. Tongues schedules every card with FSRS so each study session targets exactly the material you are most likely to lose — so you never do.