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.
That idea is older than any app. What changed is how precisely we can aim the review. Classic algorithms like SM-2 approximated the timing with fixed rules: a grade, an ease factor, a multiplier. They work. They also treat every learner and every card as roughly the same machine. You are not.
What SM-2 got right — and where it stops
SM-2, the scheduler most people met through Anki, was a breakthrough because it made spacing automatic. You grade a card; the algorithm pushes the next review out or pulls it in. For a shared deck of facts it is honest, transparent, and good enough.
Language is messier. Some words stick after two meetings. Some near-synonyms fight each other for months. A card you delayed for a week is not the same as a card you saw yesterday. SM-2’s single ease number cannot model that without a lot of manual fussing — extra fields, extra add-ons, extra ritual. Most people just live with too many reviews, or they quit.
FSRS models your forgetting, not a template
FSRS, the Free Spaced Repetition Scheduler, models forgetting directly. It tracks difficulty, stability, and retrievability for each card and fits those to how you actually remember. You can aim at a retention target instead of hoping a fixed multiplier was right. In practice that usually means fewer reviews for the same hold on the word — and a better schedule when you have been away.
TONGUES schedules every card with FSRS so each study session targets the material you are most likely to lose. The study page lines those decks up first. You do not build the scheduler, tune parameters, or maintain a separate Anki install. You generate the words you need; FSRS decides when they come back.
Spaced repetition without the right cards is still a grind. That is why TONGUES pairs FSRS with interest-first decks and comprehensible input. You review words that matter, in context, on a curve that belongs to you — so you never do the same forgotten card twice as a lifestyle.