Spaced repetition for Japanese vocabulary: the science and a practical setup

Most learners don't have a memorization problem — they have a forgetting problem. You study 20 words on Monday, feel great, and by Friday you can barely recall five of them. Spaced repetition is the single most researched fix for this, and it doesn't require more study time — just better-timed study time. This guide explains the science in plain language and then walks through a concrete setup you can run every day with Mirai Voca's review tools.

1. The forgetting curve: why cramming fails

In the 1880s, the psychologist Hermann Ebbinghaus memorized lists of nonsense syllables and tested himself at fixed intervals. The result — now called the forgetting curve — showed that memory decays fastest immediately after learning. In his data, roughly half of new material was gone within a day, and after a week only around 20–25% survived without review.

Japanese vocabulary is especially vulnerable to this decay for two reasons:

• Low similarity to English. A word like 図書館（としょかん, library）has no cognate hook the way French bibliothèque does for English speakers. Without hooks, decay is faster.
• Three writing systems. You're binding sound (としょかん), script (図書館), and meaning (library) into one memory. If any leg of that triangle weakens, the whole word feels "forgotten."

The crucial finding, though, is not that we forget — it's what happens when you review at the right moment. Each successful recall flattens the curve: after the first review you might retain the word for three days instead of one, after the second review a week, then two weeks, then a month. Reviewing just before you would have forgotten is what makes the memory durable. Reviewing too early wastes time; reviewing too late means relearning from scratch.

2. Expanding intervals: 1 → 3 → 7 → 14 → 30 days

You don't need software to exploit this effect — you need a schedule where the gap between reviews grows after each success. A classic, battle-tested ladder looks like this:

Two rules make the ladder work:

• Success moves a word up to the next, longer interval.
• Failure moves it back down — a word you miss on Day 7 goes back to the Day 1 rung, not to Day 14. Demoting failed words is not punishment; it's the mechanism that concentrates your time on exactly the words that need it.

Notice what this means for total workload: a word you know well appears only five or six times over a month. A word you struggle with appears ten or more times. Spaced repetition is, at its core, an automatic triage system for your attention.

3. Active recall beats recognition — by a lot

The second pillar is how you review, not just when. There is a huge difference between these two activities:

• Recognition: you see 食べる（たべる）next to "to eat" and think "yes, I knew that." This feels productive and is almost worthless.
• Active recall: you see 食べる alone, and you must produce "たべる — to eat" from memory before revealing the answer. The retrieval effort itself is what strengthens the memory.

In practice, test yourself in both directions, because they train different skills:

• Japanese → English (see 約束, produce "やくそく — promise"): trains reading and listening comprehension. This is the direction JLPT questions reward.
• English → Japanese (see "to lend", produce 貸す・かす): trains speaking and writing. It's noticeably harder — most learners can recognize 3–4× more words than they can produce, and that gap only closes with production practice.

One honest habit matters more than any app feature: say the answer (out loud or in your head) before you flip the card, and grade yourself strictly. "I would have gotten it" counts as a miss. Learners who grade leniently end up with hundreds of words in the "known" pile that collapse under exam pressure or in real conversation.

4. A practical setup with Mirai Voca

Mirai Voca has two tools built around exactly this science: Smart Review, which schedules words you've studied at expanding intervals and resurfaces the ones you miss, and Daily 10, which hands you a fixed, small batch of new words each day so intake stays constant. Here is a complete routine that takes 15–25 minutes a day:

• Step 1 — Pick one word source. Choose a single list that matches your level from the word index — for beginners that's JLPT N5. Resist studying from three lists at once; scattered sources make progress impossible to measure.
• Step 2 — Start every session with reviews, not new words. Open Smart Review first and clear the queue. Reviews protect what you've already paid for; new words are optional on a busy day, reviews are not.
• Step 3 — Add new words with Daily 10. After reviews, do the day's batch on Daily 10. For each word, read it aloud once, recall its meaning once, and move on — first exposure doesn't need to be perfect, because the schedule will bring it back tomorrow.
• Step 4 — Recall before revealing. On every card, produce the reading and meaning first, then check. Mark it wrong if you hesitated for more than a few seconds.
• Step 5 — Look up, don't guess. When a word confuses you (is 会う "to meet" or "to match"?), check it in the dictionary and read one example sentence. Ten seconds of context saves five failed reviews later.
• Step 6 — Same time, every day. Consistency beats volume. Anchor the session to an existing habit — with morning coffee, on the commute. The daily routine guide shows how to slot this into a 30-minute plan alongside listening and grammar.

5. How many new words per day is sustainable?

This is where most self-studiers blow up their routine. The trap: new words feel cheap on the day you add them, but every new word generates five to six future reviews. Add 50 words a day for two weeks and you've quietly signed up for 300+ reviews a day next month. Then you skip a day, the pile doubles, and you quit.

For perspective: JLPT N5 needs roughly 800 words, N4 about 1,500 total, N3 about 3,700, and N2 around 6,000. At just 10 new words a day you cover the entire N5–N4 range in under six months. 10–20 new words per day is the honest, sustainable range — and when life gets busy, the correct move is to drop new words to zero and keep doing reviews, never the reverse.

6. Leeches: the words that refuse to stick

Run any spaced repetition system for a month and you'll meet them: words you've failed eight, ten, twelve times while everything around them graduates. These are called leeches, because they suck up review time out of all proportion to their value. Common Japanese leech patterns:

• Homophone clusters: 帰る・返る・変える（all かえる — to go home / to be returned / to change）or 暑い・熱い・厚い（all あつい — hot weather / hot to touch / thick）.
• Mirror pairs: 貸す（かす, to lend）vs 借りる（かりる, to borrow）; 教える（おしえる, to teach）vs 教わる（おそわる, to be taught）.
• Visually similar kanji: 待つ（まつ, to wait）vs 持つ（もつ, to hold）— same right side, different radical.
• Abstract words with no image: 状況（じょうきょう, situation）, 場合（ばあい, case/occasion）.

Do not just keep hammering a leech — repetition without change is exactly what has already failed. Instead, after roughly the eighth miss, intervene:

• Give it a sentence. Learn 貸す inside 友達にペンを貸した（ともだちにペンをかした — "I lent my friend a pen"）. Context disambiguates what the isolated card couldn't.
• Attack the confusion directly. If two words are colliding, study them side by side once, explicitly noting the difference, rather than letting them ambush each other in the queue.
• Build one deliberate mnemonic. For 借りる（to borrow）: "you borrow, and later you must give it back — kari sounds like 'carry' it back."
• Or suspend it. Some words simply aren't ready to stick at your current level. Dropping a leech for a month and re-meeting it later — often in reading, where it suddenly makes sense — is a legitimate strategy, not a defeat.

7. Common mistakes that quietly break the system

• Skipping review days. The schedule is the method. A missed day is recoverable; a missed week means dozens of words slid back down the curve together.
• Only reviewing Japanese → English. You'll read well and freeze when speaking. Mix directions.
• Grading yourself generously. Every falsely "passed" card is a word scheduled a month out that you actually don't know.
• Adding new words while drowning in reviews. Reviews first, always. New words are a privilege the review queue has to earn.
• Treating flashcards as the whole diet. Spaced repetition builds the skeleton; reading, listening, and the kana foundations (see the hiragana guide) put flesh on it. A word met in a real sentence after being learned in review is a word you'll never lose.