べし → 〜べき / べく / べからず | Japanese Grammar

Of all the classical auxiliaries, べし is the one you cannot avoid in modern life. It runs through newspaper editorials (政府は対応すべきだ, "the government should respond"), business Japanese (然（しか）るべき対応, "appropriate measures"), and the prohibition signs on park lawns (芝生に入るべからず, "keep off the grass"). Learners typically meet 〜べき, 〜べく, and 〜べからず as three separate "advanced expressions" and never notice they are the same word. They are one classical auxiliary, べし, simply conjugating — and seeing that unifies the whole set around a single meaning core: "ought to / is to / surely will." This page is N1-level because these forms are genuinely everywhere in formal Japanese, not a museum exhibit.

One auxiliary, six shades of meaning

べし attaches to the 終止形（しゅうしけい） of a verb (for ラ変 verbs, the 連体形) and expresses a cluster of related senses. Traditional Japanese schooling memorizes them as a fixed list — 推量・意志・可能・当然・命令・適当 — but they all radiate from one idea: a strong, reasoned "this is how it is to be."

Sense	Gloss	Example flavour
推量 (conjecture)	surely will, must be	雨降るべし ("it will surely rain")
意志 (volition)	I shall, I intend to	必ず行くべし ("I shall certainly go")
可能 (potential)	can, is able to	信ずべし ("it can be believed")
当然・義務 (obligation)	ought to, must, should	学ぶべし ("one must study")
命令 (command)	shall, is to	直ちに来（きた）るべし ("come at once")
適当・勧誘 (advisability)	had better, should	休むべし ("you'd best rest")

この計画は必ず成功すべし。

kono keikaku wa kanarazu seikō subeshi

This plan shall surely succeed. (strong conjecture shading into resolve — formal / written)

若者は大いに学ぶべし。

wakamono wa ōi ni manabu beshi

Young people ought to study hard. (obligation / advice — formal)

💡

Don't memorize six unrelated meanings — hold one core: "is to / ought to / surely will," a confident, reasoned projection. Whether it lands as obligation, likelihood, or command is read off context, the way English "shall" swings between "you shall not pass" (command) and "it shall come to pass" (prediction).

The conjugation — because every branch survives

Here is why べし repays learning its full paradigm: unlike らむ or けむ, every inflected form of べし is still in daily use. It conjugates like a ク活用 adjective, with a parallel カリ活用 branch (べから / べかり / べかる) that appears before other auxiliaries.

活用形	Form	Modern reflex you already meet
未然形	べく / べから	べから + ず → べからず
連用形	べく / べかり	成功すべく努力する
終止形	べし	対応すべし
連体形	べき / べかる	するべきだ、恐るべき
已然形	べけれ	言うべけれど (literary)
命令形	—	—

〜べき — the connective form everyone uses

The 連体形べき is the reflex you meet most. On its own it modifies a noun ("the … that should be done"); with だ/である it becomes the modern predicate of obligation, 〜べきだ ("should"). This is standard N2/N1 grammar and the daily bread of journalism and argument.

政府は速やかに対応すべきだ。

seifu wa sumiyaka ni taiō subeki da

The government should respond promptly. (newspaper editorial register)

人として、守るべき約束がある。

hito to shite, mamoru beki yakusoku ga aru

As a human being, there are promises one ought to keep.

べき has also lexicalized into fixed, high-frequency expressions: 恐（おそ）るべき ("fearsome, dreadful"), 然（しか）るべき ("appropriate, fitting"), あるべき姿 ("the ideal form, how things ought to be"), 注目（ちゅうもく）すべき ("noteworthy").

彼は恐るべきスピードで成長している。

kare wa osorubeki supīdo de seichō shite iru

He is growing at a fearsome pace.

この件は、然るべき部署に相談してください。

kono ken wa, shikarubeki busho ni sōdan shite kudasai

Please consult the appropriate department about this matter. (business register)

す or する? — why すべき is not a typo

English speakers second-guess すべき and "correct" it to するべき. Both are right. Because べし attaches to the 終止形, and the classical 終止形 of the サ変 verb "to do" is す (not modern する), the historically correct form is す + べき → すべき. Modern Japanese also allows attaching to the 連体形する, giving するべき. すべき is the more formal, classical-flavoured choice; it is emphatically not an error.

私たちは今、何をすべきか真剣に考えるべきだ。

watashitachi wa ima, nani o subeki ka shinken ni kangaeru beki da

We should think seriously about what we ought to do now. (すべき from classical す — perfectly correct)

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すべき (classical す) and するべき (modern する) are both standard; すべき is not a mistake and tends to read as slightly more formal. The same goes for 愛すべき / 愛するべき, 察すべき / 察するべき.

〜べく — "so as to," "in order to"

The 連用形べく links clauses with a sense of purpose or aim — "so as to, in order to" — in a register clearly above the everyday 〜ために.

彼は試験に合格すべく、毎日夜遅くまで勉強した。

kare wa shiken ni gōkaku subeku, mainichi yoru osoku made benkyō shita

In order to pass the exam, he studied late every night. (formal / written)

べく also anchors two set idioms: 〜べくもない ("cannot possibly," 望むべくもない = "cannot even be hoped for") and 〜べくして ("as was bound to happen").

あの事故は、起こるべくして起こったと言える。

ano jiko wa, okoru beku shite okotta to ieru

You could say that accident was bound to happen. (起こるべくして起こった — 'happened as it was destined to')

〜べからず — the prohibition still guarding the lawns

The negative is built from the カリ branch: べから（未然）+ ず（negation）→ べからず, "must not, is not to." It is the crispest, most authoritative "do not" in the language — which is exactly why it still appears on real signage and in mottos.

芝生に入るべからず。

shibafu ni hairu bekarazu

Keep off the grass. (literally 'one is not to enter the lawn' — standard sign wording)

初心忘るべからず。

shoshin wasuru bekarazu

Never forget your original resolve. (Zeami's famous maxim; 忘る is classical 終止形)

The 連体形 negative べからざる ("that must not be, impermissible") modifies nouns and is common in formal writing.

それは決して許すべからざる行為だ。

sore wa kesshite yurusu bekarazaru kōi da

That is an act that can never be permitted. (formal)

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べからず is live prohibition, not archaic decoration. Read it as a stern, official "do not / is not to" — the register of signage, rules, and mottos — and note it is stronger and more impersonal than 〜てはいけない.

The distinguishing insight: it's one word, not three

The payoff of this page is compression. 〜べき, 〜べく, and 〜べからず are not three expressions to learn separately — they are the 連体形, 連用形, and negative of one auxiliary, べし, and they all carry the same meaning core: "ought to / is to." Once you hold that:

恐るべき, 然るべき, あるべき姿 stop being opaque vocabulary and become "the … that is to be feared / fitting / as it ought to be."
べからず on a park sign is obviously just the negative of the same 〜べき you read in the newspaper.
すべき is transparently the classical 終止形す plus べき — not a typo for するべき.

One auxiliary, conjugating. That is why べし is the most alive fragment of classical grammar: it never became a fossil, because it never stopped inflecting where modern formal Japanese needs it.

Common mistakes

❌ 「すべき」は「するべき」の書き間違いだ。

'subeki' wa 'suru beki' no kakimachigai da

❌ False belief: 'すべき is a typo for するべき.' It is not — すべき is the classical 終止形す plus べき.

✅ 政府は対応すべきだ。

seifu wa taiō subeki da

✅ 'The government should respond.' Both すべき (classical す) and するべき (modern する) are standard.

❌ 芝生に入らないべし。

shibafu ni hairanai beshi

Wrong negation — you don't negate the verb before べし. The negative lives in べし itself: べから + ず.

✅ 芝生に入るべからず。

shibafu ni hairu bekarazu

Keep off the grass. (negation is carried by べからず, not by 入らない)

The negative of a べし sentence is 〜べからず (prohibition) or 〜べきではない (softer "should not") — never a plain negative verb plus べし.

❌ 明日は早く起きるべきだと思うから、そろそろ寝るべきだわ。

ashita wa hayaku okiru beki da to omou kara, sorosoro neru beki da wa

Register overload — べきだ is formal, prescriptive 'ought.' For your own casual plans it sounds preachy; use 〜なきゃ / 〜たほうがいい.

✅ 明日早いから、そろそろ寝なきゃ。

ashita hayai kara, sorosoro nenakya

I've got an early start, so I'd better get to bed. (natural casual advice to oneself)

❌ 食べべきだ

Wrong stem — べき attaches to the 終止形 (連体形). For 食べる that's 食べる, giving 食べるべき, not 食べ + べき.

✅ 食べるべきだ

taberu beki da

ought to eat

❌ 合格するために、彼はべく努力した。

gōkaku suru tame ni, kare wa beku doryoku shita

Doubled and detached — べく is not a free adverb. It attaches to a verb's 終止形: 合格すべく努力した.

✅ 合格すべく努力した。

gōkaku subeku doryoku shita

He made efforts so as to pass. (formal)

Key takeaways

べし attaches to the 終止形 and means, at its core, "ought to / is to / surely will," flexing into obligation, likelihood, potential, and command by context.
〜べき (連体形) → するべきだ / すべきだ ("should"), plus fixed 恐るべき, 然るべき, あるべき姿; it is standard journalism and N1 grammar.
〜べく (連用形) → "so as to, in order to" (合格すべく), and idioms 〜べくもない, 〜べくして.
〜べからず (べから + ず) → live, authoritative prohibition on real signage (芝生に入るべからず, 初心忘るべからず); 連体形べからざる.
すべき is not a typo — it is the classical 終止形す plus べき, sitting beside modern するべき.
These are one auxiliary conjugating, not three separate expressions — which is why べし is the most alive scrap of classical grammar in the modern language.

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