Spaced Repetition Explained Simply: How to Remember Vocabulary for Good

You learn a new word, feel sure you know it, and three days later it is gone. That is not a personal failing. It is how memory works. Spaced repetition is the most practical tool we have for working with that fact instead of against it.

This article explains the idea in plain language, shows how review schedules work, and gives practical tips for using it to learn vocabulary.

Forgetting is normal, and predictable

In the 1880s the German psychologist Hermann Ebbinghaus ran memory experiments on himself, learning lists of nonsense syllables and testing how much he retained over time. He described what is now called the forgetting curve: memory drops quickly at first and then more slowly.

The second important observation was that each time you successfully review something, it fades more slowly afterwards. Every review flattens the curve a little.

Put those two facts together and you get the core idea:

Review each item just before you would forget it, and space each review further apart than the last.

What “spaced” means in practice

Compare two ways of studying the same word for the same total time:

• Massed practice (cramming): look at the word five times in a row today.
• Spaced practice: look at it once today, once tomorrow, once in three days, once next week and once in a few weeks.

Cramming can feel more effective in the moment, because the word is fresh. But the spaced version usually leads to much better long-term retention. Researchers call this the spacing effect, and it has been found across many kinds of material, ages and settings.

There is a related idea worth knowing: retrieval practice. Trying to recall an answer (looking at the word and thinking “what does this mean?”) strengthens memory more than simply rereading it. Flashcards combine both: you retrieve, and you do it at spaced intervals.

How a review schedule works

A spaced repetition system (SRS) decides when you see each card again based on how well you remembered it last time. Here is a simplified example of how intervals might grow for a card you keep getting right:

Review	You remembered?	Next review in
1st (learning)	Yes	1 day
2nd	Yes	~3 days
3rd	Yes	~1 week
4th	Yes	~2–3 weeks
5th	Yes	~1–2 months
6th	No	back to a short interval

The exact numbers differ between systems, and good systems adjust them to you. The pattern is what matters: intervals grow when you succeed and shrink when you fail. Easy words quickly drift out of your way; difficult words keep coming back until they stick.

A short history of the algorithms

• The Leitner system (1970s): a physical method using boxes of paper cards. Cards you get right move to a box you review less often; cards you get wrong go back to the first box.
• SM-2 (late 1980s): an algorithm from the SuperMemo software that calculates intervals using an “ease” value for each card. Variations of it were used in many flashcard apps for decades.
• Newer models: some modern systems estimate, for each card, how likely you are to remember it on a given day, and schedule the review for when that probability drops to a target level. Anki, for example, now offers an optional scheduler of this kind called FSRS.

You don’t need to understand the maths to benefit. You just need to show up when the cards are due.

Why spaced repetition is so good for vocabulary

Vocabulary is a perfect fit:

• There is a lot of it. You need thousands of words, and you can’t keep them all fresh by rereading.
• Words are discrete. Each one can be a card.
• Forgetting is uneven. Some words stick on the first try; others take ten exposures. An SRS spends your time where it’s needed.

The result is efficiency. A few minutes of daily review can maintain a deck of hundreds or thousands of words, because on any given day only a small fraction of them are due.

Making better cards

The quality of your cards matters as much as the schedule.

Add context. A single word with a single translation is fragile. A word inside the sentence where you met it is much easier to remember and teaches you how the word is used. For example, a card for cửa sổ (window) is stronger with the sentence Lan uống cà phê bên cửa sổ (“Lan drinks coffee by the window”).

Keep each card to one thing. If a card tests three meanings at once, you will never know which one you forgot.

Prefer words you’ve met in real text. Words you encountered in a story come with a memory of where you saw them, which gives the card an extra hook. This is one reason learning vocabulary from reading tends to beat memorising lists.

Don’t add everything. Every card is a future review. Add words you’ve seen more than once or that feel genuinely useful.

How much review is enough?

Review load grows with the number of new cards you add, not with time. A simple rule: if your daily reviews start to feel like a chore, add fewer new cards for a while. The backlog will settle.

For a ten-minute daily routine, a reasonable starting point is something like:

• 5 or so new words per day from your reading
• all due reviews, every day

Adjust from there. Consistency matters far more than volume. Skipping reviews for a week creates a pile that is discouraging to clear; doing a few minutes every day keeps it light. (Our guide to learning a language in 10 minutes a day shows how reviews fit into a short session.)

Honest grading

When you review a card, be honest with yourself:

• If you hesitated for a long time or got it half right, mark it as hard or wrong.
• If you knew it instantly, mark it as easy.

Being generous with yourself feels nice but pushes cards too far into the future, and you’ll meet them again only after you’ve forgotten them.

Common misunderstandings

• “Spaced repetition replaces reading and listening.” It doesn’t. It keeps words in memory, but you still need lots of real input to learn how words behave, to build grammar intuition and to understand speech.
• “I need to review every day forever.” You need to review on the days cards are due. In practice, with a daily habit, that’s most days, but each session can be short.
• “If I forget a card, I failed.” Forgetting is part of the process. A card coming back sooner is the system doing its job.

How DailyFluent uses spaced repetition

In DailyFluent, every word you tap while reading is saved with the sentence it came from and scheduled for review automatically. You don’t type anything; the deck builds itself from what you actually read. Reviews happen in the same short daily session as your reading. The app is launching soon, first for English and Vietnamese learners.

Summary

• Memory fades in a predictable way, and each successful review slows that fading.
• Spaced repetition schedules reviews at growing intervals, just before you would forget.
• It’s ideal for vocabulary, especially when cards include context from real reading.
• Add fewer cards than you think, grade honestly and review a little every day.