Spaced repetition, explained without the jargon

You learn a new word on Monday. By Thursday it's gone. You meet it again, it feels familiar, and this time it lasts a week. That pattern is the whole idea behind spaced repetition: each successful recall makes a memory last longer, so the gaps between reviews can grow.

The forgetting curve

In the 1880s the German psychologist Hermann Ebbinghaus memorised lists of nonsense syllables and tested himself on how quickly he forgot them. His results are usually drawn as the forgetting curve: memory drops fast at first, then more slowly. His other key finding was that reviewing material resets the curve, and each review makes the next drop gentler.

Ebbinghaus studied himself and nonsense syllables, so his exact numbers aren't a law of nature. But the general shape has been confirmed many times since, and it's the reason cramming works for tomorrow's test and fails for next month's conversation.

From shoe boxes to software

In the 1970s the German science writer Sebastian Leitner popularised a simple system with a box of cards divided into compartments. Cards you got right moved to a compartment you reviewed less often; cards you got wrong went back to the first one. It's spaced repetition with no software at all, and it still works.

Later, software made the schedule more precise. Piotr Woźniak's SM-2 algorithm, developed for the SuperMemo program in the late 1980s, became the ancestor of many modern flashcard schedulers. The core logic is easy to describe:

1. When you remember a card, the next review is pushed further into the future.
2. The better you remembered it, the bigger the jump.
3. When you forget a card, it comes back soon and the gaps shrink again.

What the buttons mean

Most apps ask you to rate each card. Typically:

• Again: you didn't remember it. You'll see it again in a minute or two.
• Hard: you got it, but it took effort. The gap grows only a little.
• Good: you remembered it normally. The gap grows steadily, for example 1 day, then 3, then longer.
• Easy: instant recall. The gap grows faster.

In Wordly's flashcards, for instance, a new card you rate Good comes back the next day, then after three days, and then at intervals that keep growing. Cards that you keep getting right eventually count as mastered but still return occasionally so they don't fade. It's a deliberately simple version of the SM-2 idea, not anything magical.

The mistakes that make reviews pile up

Adding too many new cards. Every new card creates future reviews. Ten new words a day sounds modest, but after a month you might face a hundred reviews a day. Add new cards at a pace you can sustain on your busiest day, not your best one.

Rating optimistically. If you hesitate for five seconds and then guess, that's Hard, not Good. Honest ratings are what make the schedule work.

Studying words with no context. A card that says aprender → to learn is fine. A card that also shows Aprendo palabras nuevas cada día is better, because you remember the word as part of a sentence. Even better is meeting the word again while reading.

Skipping a week, then giving up. A backlog feels awful, but it's just a queue. Do your normal amount each day and it clears. There's more on that in building a habit that survives busy weeks.

Is it worth it?

For the core vocabulary of a language (the first few thousand words), spaced repetition is one of the most time-efficient tools available. It doesn't teach you to understand conversations or enjoy a novel; reading and listening do that. But it makes sure the words you need are there when you get to them.