Why we forget words — and how spaced repetition helps
Memory fades fast, and more slowly each time you recall a word. Here is the science behind spaced repetition and how Duendillo uses it, in plain words.
By the Duendillo team · ·5 min read
You learn la ventana on Monday. On Thursday you look at a window and the word is gone. That is not a lack of talent: it is how memory works for everyone. The good news is that forgetting follows a pattern, and a pattern can be planned around.
The forgetting curve
In the 1880s the German psychologist Hermann Ebbinghaus learned lists of nonsense syllables and tested himself after different delays. He found that memory drops steeply at first — much of a new list was gone within a day — and then more and more slowly. This shape is called the forgetting curve.
He also noticed something more useful: a list he relearned was forgotten more slowly each time, and repetitions spread over several days worked better than the same number in one sitting. A word you have recalled three times on different days lasts far longer than a word you have read ten times in one evening.
Two effects that make it work
Researchers have confirmed two effects again and again since then:
- Spacing. Reviews spread over days beat the same number of reviews crammed into one session. A short break, with a little forgetting in it, makes the next review count more.
- Recall. Trying to remember an answer strengthens memory more than reading it again. This is why it helps to answer before you look, even when you are not sure.
Spaced repetition combines the two: you recall a word just before you would forget it, and each time the gap to the next review grows — a day, then a few days, then weeks, then months.
From fixed schedules to personal ones
The first systems used fixed rules. In the 1970s Sebastian Leitner sorted paper flashcards into boxes: a card you knew moved to a box you reviewed less often, a card you missed went back to the first box. In 1987 Piotr Woźniak wrote SM-2 for SuperMemo, the formula many flashcard apps still use. Both apply the same fixed rules to every learner; SM-2 adapts to each card only through a simple “ease” number.
FSRS (Free Spaced Repetition Scheduler) is a newer, open-source model trained on millions of real reviews. For each word it keeps three numbers:
- stability — how many days it takes for your chance of recalling it to drop to 90%;
- difficulty — how hard this word is for you;
- retrievability — your chance of recalling it right now.
After every answer FSRS updates these numbers. A review is due when the chance of recalling the word falls to about 90%: early enough that you still remember it with a little effort, late enough that the effort strengthens the memory.
How Duendillo uses it
Duendillo runs FSRS for every word you learn, and in both directions: recognizing la ventana when you read it and producing it when you need it are two separate memories, so each has its own schedule.
There are no “easy / hard” buttons to press. The exercise itself tells the model how it went: a wrong answer, an answer that was almost right (cancion without its accent, a slip in one letter), a correct one, or a correct one given faster than usual.
Grammar is different. A rule such as the difference between ser and estar is not something you forget overnight like a single word; it is a skill you either have or are still building. For grammar Duendillo uses a second model, Bayesian Knowledge Tracing, which estimates from your answers how well you know each skill. Skills you have mastered step aside, and the shaky ones come back more often.
Both models work in the background. You only see the next exercise — usually one you can answer, but not without thinking.
Five habits that help
- A little every day. Ten minutes a day beats an hour on Sunday, because the reviews land on the right days.
- Answer before you look. The effort of recalling is what makes a word stick, even when the answer comes out wrong.
- Do not fear mistakes. A word you miss comes back at the end of the session and again soon after; that is the system working.
- Let words come back. Forgetting a little between reviews is not a failure — a memory you rebuild is stronger than one you never had to.
- Learn words in sentences. Abre la ventana, por favor gives a word more to hold on to than ventana alone.
Forgetting is not the enemy of learning. It just needs good timing — and that is the part Duendillo takes care of.