How spaced repetition works, and why the gap keeps growing

The curve nobody escapes

In 1885 Hermann Ebbinghaus memorised lists of nonsense syllables and tested himself at intervals. What he found has been replicated ever since: memory decays fast at first, then slower. An hour after learning something you have lost a good part of it; a day later, more; after a week, most of it is gone, unless something happened in between.

The something is retrieval. If you are asked for the item again before it fades and you manage to produce it, the curve resets and, crucially, flattens. The next decay is slower. Ask again just before it fades a second time and it flattens further. After a handful of well-timed retrievals, an item that would have vanished in a week now lasts for months.

Two details matter. The retrieval has to be a real one: you produce the answer, you do not look at it. And the timing has to be late enough to be an effort. Repeating a word ten times in a row feels productive and does almost nothing, because the ninth repetition asks nothing of memory. The same ten repetitions spread over a month, each one a little harder than the last, are what make a word permanent.

Why the gap grows

Every successful retrieval makes the memory more durable, so the next review can wait longer. This is why every spaced repetition system, from paper flashcard boxes to modern algorithms, follows the same shape: short gaps at the start, longer and longer gaps afterwards. The first review comes in minutes, the next in hours, then days, then weeks. Eventually an item is asked for a few times a year, which is enough.

The reverse also holds. If you fail a retrieval, the memory was weaker than the schedule assumed, and the gap has to shrink. A good system does not punish you for that; it simply asks again sooner.

How the course schedules

Every word, every form of a word and every grammar rule has a level from 1 to 7, and the level is the gap. A word at level 1 comes back in ten minutes. Level 2 waits an hour, level 3 a day, level 4 three days, level 5 a week and level 6 sixteen days. Level 7 means the item is mastered: it comes back after a month, and from there the gap doubles with each correct answer, up to a year.

A correct answer when the item is due moves it up one step. A correct answer before it is due changes nothing: no level, no date, because answering early proves little. A miss halves the level. A word at level 6, due every sixteen days, drops to level 3 and comes back tomorrow; if it slips again during the same session, it halves once more. Nothing is ever wiped. There is no card that goes back to zero and no deck that gets reset for a week of absence. If you return after two weeks away, everything that came due is waiting, and a level only ever drops when you answer wrong.

Coming back late works in your favour. Say a level-3 word was due after a day and you answer it correctly only after twenty. Your memory has just shown that it holds the word for at least twenty days, so the word does not climb a single step: it lands on the highest step whose wait it has already sat out, here level 6. The one step it never jumps to is 7. Mastery always takes a correct answer on the step just below it.

With this ladder, an item goes from its first appearance to mastered in about a month and six correct answers on time. That is deliberate. Getting a word right three times in an afternoon is short-term memory; getting it right after sixteen days without seeing it is knowing it. And mastered is not for ever: one mistake brings the level down, and the word keeps coming back until it has earned the top step again.

The same schedule for grammar

Most systems space words only. Here a rule is an item like any other, with the same ladder. The rule for the locative after u and na has a level, comes due, and is tested every time you write a sentence that needs it. If you produce the wrong ending, it is that rule which halves and comes back tomorrow, together with the specific form you missed.

What this looks like day to day

Most of your time in the course is repetition, not new material, and that is the point. Each session begins with what is due: the words, forms and rules whose gap has run out today, the weakest first. You write them, in sentences, until nothing due is left. New lessons come after. Because the strong items drift off towards a month and a year, the daily pile stays made of the things you are still unsure of. The words you know stop asking for your time.

You can look up the level of everything you know at any moment, word by word and rule by rule. It is the honest version of a progress bar: not how many lessons you have opened, but how much of each thing is actually yours.