The forgetting curve: why you forget words and how to fix it

You study 30 new words in the evening. You feel confident. A week later, you sit down to review and maybe 5 of them come back. The rest have vanished, as if you never looked at them. This is not a personal failing or a sign that you're bad at languages.

It's how human memory works. Your brain is built to forget things it doesn't encounter repeatedly, and new vocabulary is especially vulnerable. The good news is that forgetting follows a predictable pattern, and once you understand that pattern, you can work with it instead of against it.

In the 1880s, a German psychologist named Hermann Ebbinghaus decided to study memory by experimenting on himself. He memorized lists of nonsense syllables (meaningless combinations like "DAX" and "BUP") and then tested how many he could recall after various time intervals. What he found was striking: memory doesn't fade gradually. It drops off a cliff.

Within the first hour, he'd lost roughly half of what he'd learned. After 24 hours, about 70% was gone. After a week, the number climbed to around 90%. He plotted these results on a graph that became known as the forgetting curve. The shape is exponential: steep at first, then flattening out as the remaining memories stabilize. Ebbinghaus published these findings in his 1885 monograph Memory: A Contribution to Experimental Psychology, and the core result has been replicated many times since.

But Ebbinghaus also noticed something encouraging. When he reviewed the material at the right moment, before the memory had fully decayed, the curve flattened. Each review made the next bout of forgetting less severe. The memory lasted longer before it needed refreshing again. This observation is the foundation of every spaced repetition system that exists today.

When you learn a new vocabulary word, the same exponential decay applies. You encounter "Hund" (German for dog), understand it, and your brain files it away. Without review, that memory weakens rapidly. But if you see the word again the next day, the memory trace gets reinforced and lasts longer. Review it again after three days, and it holds for a week. After that, you might not need to see it again for a month. Each well-timed review extends the next interval — a word that needed daily review in week one might only need monthly review by month three.

Without structured review, learners end up in a frustrating loop: re-learning the same words over and over because nothing sticks. The problem isn't the learning. It's the absence of review at the right times. You keep resetting to zero because nobody told you when to look at those words again.

Spaced repetition systems (SRS) automate the review timing that Ebbinghaus identified as critical. Instead of reviewing everything on a fixed schedule, an SRS calculates when each individual word is about to fade and shows it to you at that moment. The SM-2 algorithm, originally developed for SuperMemo and now used by tools like Anki and Langadoo, handles this calculation. After each review, you indicate how well you remembered. Easy words get pushed further into the future. Hard words come back sooner.

A 2006 meta-analysis by Cepeda et al. reviewed 254 studies comparing spaced and massed practice. The conclusion was unambiguous: distributing practice over time produces significantly better long-term retention than concentrating it in a single session. The effect was consistent across different age groups, materials, and task types. For vocabulary learning specifically, spaced practice means you retain more words with less total study time.

The practical result is that your daily review session stays manageable even as your total vocabulary grows into the thousands. You might review 30 to 50 cards per day, and that's enough to maintain a vocabulary of several thousand words. Read more about how spaced repetition works for language learning.

Massed practice, commonly known as cramming, can produce short-term results. You study intensively for two hours before a test and perform reasonably well. But the forgetting curve is at its steepest for crammed material. Within days, most of what you "learned" is gone. This is because cramming creates the illusion of learning without building durable memory traces. You recognize the word while it's still in working memory, but it never transfers to long-term storage.

Distributed practice (spacing your study out) feels slower at first. You won't ace a quiz the morning after a single short session. But over weeks and months, spaced learners consistently outperform crammers on retention tests. This is one of the most replicated findings in all of cognitive science. If your goal is to remember vocabulary for years rather than hours, cramming is the worst possible strategy.

Not all memories decay at the same rate. Words learned in rich context, from a video you were watching, a story you were reading, or a conversation you were having, have shallower forgetting curves than words learned from isolated word lists. The reason is that contextual learning creates multiple retrieval paths. When you learn "lluvia" by reading it on a flashcard, you have one cue: the translation "rain." When you learn it from a scene in a movie where someone is running through a storm, you have visual cues, auditory cues, emotional associations, and the surrounding words.

All of those extra connections make the memory more resistant to decay. They give your brain more ways to find the word when you need it. This is why building vocabulary from context is so much more effective than drilling word lists. The forgetting curve still applies, but it starts from a stronger position.

Every feature in Langadoo is designed to flatten the forgetting curve. Vocabulary is collected from real YouTube videos and texts, so words arrive with context, audio, and sentence examples already attached. The SM-2 algorithm schedules reviews at optimal intervals, and audio clips let you hear words spoken by native speakers, adding an auditory memory trace. AI-generated stories give you repeated encounters with your saved words in new contexts, strengthening the memory from multiple angles.

The combination of contextual learning and spaced repetition addresses both sides of the retention problem: strong initial encoding and well-timed review. If you're looking for a tool built around these principles, see how Langadoo compares to Anki or read about how many words you need to be fluent.

• Ebbinghaus, H. (1885). Memory: A Contribution to Experimental Psychology. The original study of the forgetting curve and spaced review. Read more
• Cepeda, N. J. et al. (2006). "Distributed practice in verbal recall tasks." Psychological Bulletin, 132(3), 354-380. A meta-analysis of 254 studies confirming the advantage of spaced over massed practice. View article
• Kornell, N. (2009). "Optimising learning using flashcards." Applied Cognitive Psychology, 23(9), 1297-1317. Research on effective flashcard strategies and spacing. View article
• Bahrick, H. P. et al. (1993). "Maintenance of foreign language vocabulary and the spacing effect." Psychological Science, 4(5), 316-321. Demonstrated that spaced review sessions produced better long-term retention of foreign language vocabulary than massed sessions. View article