What Is Spaced Repetition, and Why Does It Work?

The problem: forgetting is the default

Memory decays on a curve. Within a day of learning something new, most of it is gone; within a week, nearly all of it, unless you meet it again. This is the forgetting curve, first measured by Hermann Ebbinghaus in the 1880s, and it is why cramming feels productive and then fails you two weeks later.

The useful discovery is not that we forget. It is that each time you successfully recall something just before you would have forgotten it, the memory comes back stronger, and the next forgetting happens more slowly.

The fix: review at expanding intervals

Spaced repetition schedules each review at the moment it does the most good: right before you would forget. Get a word right and the next review moves further out, a day, then three, then a week, then a month. Get it wrong and the schedule contracts, showing it again soon.

Two well-studied effects do the work. The spacing effect: material reviewed across spread-out sessions is retained far better than the same time spent in one sitting. The testing effect: actively recalling an answer strengthens memory more than rereading it ever does. Flashcards combine both.

What it looks like in practice

You study a small stack of due cards each day. Each card shows a prompt, you recall the answer, flip, and grade yourself. The scheduler does the rest: easy cards drift out to monthly reviews, hard cards stay close until they stick.

The daily commitment is small and stays small. Ten minutes a day of due reviews beats an hour of weekend rereading, because every minute is spent on exactly the cards you were about to forget.

Why it fits language learning

Vocabulary is thousands of small, independent facts, which is precisely the shape of knowledge spaced repetition handles best. A word you keep meeting at expanding intervals becomes a word you produce without thinking, and that automaticity is what conversation runs on.

It works best when the cards are worth remembering: real phrases you will actually say, with audio so the word you learn is the word you hear. That is how we build every Kioki deck.