Making 英単語 stick

Start here if you read nothing else.

If they want one recommendation and no configuration

iKnow! for the daily habit (¥9,530/yr, Japanese interface, genuinely adaptive, and free if they already pay for DMM英会話). Netflix with Japanese and English subtitles both switched on. One written sentence per new word.

If they're a tinkerer like you

Anki with FSRS at 0.85 to 0.90 desired retention, fed by Yomitan (free, and it now handles English) or Language Reactor pulling words out of whatever they're already reading and watching. Same input and output layers on top.

Three questions change the answer completely. Ask them before handing over a tool.

1. How many words do they already have?

The advice inverts around roughly 3,000 word families. Below that line, frequency lists and bulk SRS are the fastest path, because sheer forced exposure is what's missing. Above it, frequency lists go stale quickly and what's missing is nuance, register, and collocation, which only context teaches. Most Japanese adults who finished high school English are already near or past that line, which is why grinding another 単語帳 so often fails them.

2. What is the target?

Coverage numbers make this concrete. About 3,000 word families gives roughly 95% coverage of TV and TOEIC material, which is enough to watch and to work. Around 8,000 to 9,000 is what novels and general written English need at 98%, the point where reading gets pleasant and incidental learning starts to work. Those are very different amounts of effort, and the second one is not reachable by flashcards.

3. Is the damage receptive or productive?

Someone who reads technical English comfortably but freezes in meetings does not need more vocabulary. They need output practice. Someone who can't follow a podcast may also not have a vocabulary problem: katakana-locked pronunciation can block recognition of words they already know on paper. Both feel like "I need more vocabulary" from the inside, and neither is fixed by a bigger deck.

Percentages are of total study time. The order is by time spent, not by importance. Layer four has the strongest experimental support of anything here.

Plus one thing you do once and never again

A short structural pass on Latin and Greek roots and affixes. The twenty most common English prefixes account for 97% of all prefixed words, and explicit affix teaching shows measurable vocabulary gains. This is the closest real analogue to WaniKani's radicals, and in Japan the obvious vehicle is 『英単語の語源図鑑』, a genuine bestseller, illustrated and cheap. Treat it as a few weeks of decoding practice rather than an ongoing daily habit.

Ranked by fit for a Japanese speaker rather than by general popularity. Prices are approximate and worth re-checking.

If they go the Anki route, two settings matter

Turn on FSRS. It has been Anki's built-in scheduler since v23.10 and it beats the older SM-2 algorithm on 99.6% of user collections, needing roughly 20 to 30% fewer reviews for the same retention. Worth saying out loud to a WaniKani user: that also means it beats WaniKani's fixed Apprentice to Guru to Master ladder, which is a hardcoded schedule with no per-user fitting.

Set desired retention to 0.85 or 0.90 rather than higher. The 90% default is a convenience, not an optimum. Workload against retention is a U-shaped curve, and pushing toward 95% buys small accuracy gains for a steep rise in daily reviews. For heavy vocabulary loads, lower is the right trade.

Pre-made decks, if starting from a book they trust

Every major Japanese wordbook has a shared Anki deck: 英単語ターゲット1900, システム英単語, DUO 3.0 with audio, 出る単特急 金のフレーズ for TOEIC, でる順パス単 for 英検. Quality is inconsistent, and 鉄壁 in particular has no reliably good free deck, which is a long-running complaint. For a cleaner modern alternative, the NGSL is free and openly published at 2,809 words covering about 92% of general text. Add the TOEIC Service List and the pair reaches roughly 98.5% coverage of TOEIC material.

The "capture it while I'm reading or watching" requirement. Most people skip this layer, and it decides whether card-making stays a chore.

Don't mine a word you'll never meet again. The sentence-mining consensus is explicit about excluding one-off specialist terms. Mine what's frequent, what blocks comprehension, or what matters to their work, and nothing else.

This is where generic English-vocabulary advice goes wrong, and why a tool built for American high schoolers is the wrong recommendation.

Katakana has already taught them the wrong word

Loanwords are not accented English. Many have drifted in meaning, so prior knowledge here is a liability rather than a head start. These words need correction framing ("not X, actually Y") rather than first-exposure framing, and they need extra repetitions, because a wrong association is already competing for retrieval.

Katakana also locks in the wrong pronunciation

Every katakana mora but ん ends in a vowel, so text becomes テキスト, four morae for a one-syllable word. When a word is learned through its katakana form first, which is how most English words reach a Japanese adult, that shape becomes the entrenched entry in memory. They can recognise the word in print but not hear it in speech, and their own production reproduces the katakana shape.

That settles the card design. Audio on every card, with no exceptions, because a text-and-meaning card actively reinforces the katakana entry. Where the katakana diverges structurally, in vowel length, consonant clusters, final consonants, or stress placement, flag it.

Collocations are the highest-error area

Japanese pedagogy names this repeatedly as a distinct failure mode. Building English phrases by direct translation from Japanese produces grammatical but unnatural combinations, like "strong rain" for heavy rain. You can know exactly what a word means and still misuse it in combination. That's why システム英単語's ミニマルフレーズ design, which embeds words in two-to-four-word phrases rather than isolating them, is a better card model than a bare word list, and why the unit on the card should be a phrase.

Phrasal verbs get avoided, not forgotten

Dagut and Laufer found that learners with non-Germanic first languages systematically prefer single-word Latinate verbs, choosing "investigate" over "look into." That finding has been directly replicated for Japanese learners. The cause is curricular under-exposure rather than aptitude. Yasuda's work in TESOL Quarterly found that teaching the conceptual metaphor behind the particle, treating up and out and off as spatial schemas, measurably helped Japanese learners. Rote-carding "look up = 調べる" is a weak encoding for this word class specifically.

Articles and prepositions won't yield to memorization

Japanese has no article system at all, and learners tend to map English prepositions one-to-one onto 助詞, assuming で always means "at" or "in" or "with." Research on Japanese learners finds preposition errors track prototypicality: the core spatial sense gets acquired and the extended senses don't. Neither of these is a vocabulary problem and neither is fixed by a deck. They're exposure problems. Point them at layer three.

Worth quoting directly.

That last figure is the one to lead with. It moves the request from "I need more words" to "I need to activate the words I have," which points at a completely different set of tools.

Commonly recommended, and not worth their time.

Pick by who they are rather than by what sounds most rigorous.

Key references

• Nation (2006), "How Large a Vocabulary Is Needed for Reading and Listening?" for the 95% and 98% coverage thresholds and the word-family counts behind them.
• Webb (2007), "The Effects of Repetition on Vocabulary Knowledge", run on 121 Japanese EFL learners, for the ten-encounter figure and for meaning knowledge lagging form knowledge.
• Hulstijn and Laufer (2001), the Involvement Load Hypothesis: composition beats gap-fill beats reading.
• Roediger and Karpicke (2006), "The Power of Testing Memory" for 61% against 40% at one week, and for why the effect is invisible in the short term.
• Dizon and Thanyawatpokin (2021), "Language learning with Netflix" on dual subtitles beating L2-only captions.
• FSRS: the optimal retention and the FSRS benchmark, on why 0.90 is a default rather than an optimum.
• Yasuda (2010), "Learning Phrasal Verbs Through Conceptual Metaphors", on Japanese learners specifically.
• Kobayashi, "Avoidance of Phrasal Verbs by Japanese Learners of English" in CELES, the replication of Dagut and Laufer.
• The New General Service List and the TOEIC Service List, both free and openly published from a modern corpus.
• SSS英語多読研究会 for Japan's extensive-reading movement and its three rules.
• Refold on learning words with Anki, source of the five-to-ten-cards-a-day and 30% time guidance.
• AnkiWeb forums, "1671 reviews / burnout", the burnout thread and what actually fixed it.