Why do we forget things, and how can spaced repetition fight the forgetting curve? Dive into the research.
The forgetting curve
In the 1880s Hermann Ebbinghaus measured how quickly he forgot newly learned material and found something uncomfortable: memory decays fastest immediately after learning. Much of what you study today is gone within days unless something interrupts that decline.
The interruption is retrieval. Each time you successfully pull a memory back, the curve resets and flattens — the same item is forgotten more slowly than it was before.
Why spacing beats cramming
Reviewing the same word five times in one sitting produces a strong feeling of mastery and very little durable memory. Spreading those five reviews across two weeks feels harder and produces far more. Psychologists call this the spacing effect, and it is one of the most reliably reproduced findings in the study of learning.
The difficulty is the point. A review that requires effort strengthens the memory; a review that is effortless mostly confirms you already knew it.
Timing the next review
The useful moment to review is just before you would have forgotten. Recall an item too early and you gain little; too late and you are relearning from scratch. Spaced-repetition systems approximate that moment by widening the gap each time you answer correctly, and shortening it sharply when you do not.
This is why a well-scheduled review session shows you a mix you did not choose: yesterday's mistakes alongside words you last saw a month ago.
Making it work in practice
Keep sessions short and frequent — spacing only works if the reviews actually happen. A backlog of overdue items is the usual reason people abandon the method, so it is better to add new words slowly than to build a queue you dread.
Recall actively before you check. Looking at a word and thinking 'yes, I know that one' is recognition, not retrieval, and it does not produce the same strengthening effect.