“〜beki・beki da”: Should / Ought To 〜 (a moral, self-evident obligation) [JLPT N1]

Content by NTJ Kuma · Nihongo to Japan editorial team · Updated July 27, 2026

「人の悪口を言うべきではない」 — the moral “ought,” stronger than mere advice.

「約束は守るべきだ」 — by principle, this is what one ought to do

「一度した約束は、守るべきだ」 (a promise once made ought to be kept) — 「〜べきだ」 expresses “ought to ～, should ～”: subjectively asserting “this is what one ought to do,” based on principle, common sense, duty. It's stronger and more principled than 「〜たほうがいい (had better …)」. This guide covers 「べき」's use, attachment (する→すべき exception), and separates it from 「なければならない」「はず」 (N2).

Core: ought to ～, should ～ (a strong assertion based on principle)

「(verb dictionary form) ＋ べきだ」 = “ought to ～, should ～, is right to ～” — based on principle, duty, social common sense, asserting “(no matter what) this is what one should do”:

• 学生は勉強するべきだ (students ought to study).
• 間違ったら、素直に謝るべきだ (if you're wrong, you ought to apologize honestly).

Feel: “ought to …／should …” — the core is “a subjective assertion ‘this is right' based on principle,' with a stronger normative, admonishing sense. The negative 「〜べきではない」 = ought not to ～ (そんなことを言うべきではない = you shouldn't say such a thing). A formal, principled expression.

Attachment & the 「すべき」 exception

See it in context

• 困っている人がいたら、助けるべきだ。(If someone's in trouble, you ought to help.)
• 自分の意見は、はっきり言うべきだ。(You ought to state your own opinion clearly.)
• こんな大事なことを、隠すべきではない。(Something this important shouldn't be hidden.)
• 若いうちに、いろいろ経験すべきだ。(While young, you ought to experience many things.)
• あの時、もっと努力すべきだった。(I should have tried harder back then.)

⚠️ Easy traps to avoid

① 「べきだ」 (ought to, by principle) vs 「なければならない」 (must, have to) differ in tone. Both mean “should do,” but 「べきだ」 is a subjective assertion ‘this is right' based on ‘principle, common sense, duty' (約束は守るべきだ = one ought to keep promises — by principle); 「なければならない」 is ‘must, have to' (must do out of rules, duty, circumstance, more objective compulsion) (明日までに出さなければならない = must submit by tomorrow — a rule/deadline). For “(by principle) one ought to …” use べきだ, for “(by rule/circumstance) must …” use なければならない. べき leans “the ought of principle,” なければならない “the must of compulsion.”

② 「する」's attachment is 「すべき」 (exception). The most-erred attachment. する→すべき (the classical 「す」, 努力すべき, 注意すべき, 対応すべき) — though 「するべき」 isn't wrong, 「すべき」 is commoner, more natural, more tested. Same for Sino-Japanese verbs: 勉強すべき, 検討すべき. See a する-verb ＋ べき and prefer 「すべき」.

③ 「べき」 isn't the conjectural 「はず」 — don't confuse them. Both may render as “should,” but differ. べきだ = ought to ～ (should do, by duty/principle) (謝るべきだ = ought to apologize, ‘it's right/a duty to do'); はずだ = should ～ (a conjecture, expectation, ‘by reason it will be …') (もう着くはずだ = should be arriving soon, ‘a conjecture, expectation'). For “ought to do (duty)” use べき; for “conjecture, expectation” use はず. English/Chinese “should” has both senses; Japanese separates them clearly — don't confuse them.

🔄 Compare: べきだ／なければならない／はずだ／たほうがいい

💡 How to remember this

「〜べきだ」 = ought to ～, should ～, its core “a subjective assertion ‘this is what one should do,' based on principle, common sense, duty” (約束は守るべきだ; 間違ったら謝るべきだ). Three points: ① 「べきだ」 leans ‘the ought of principle,' differing in tone from 「なければならない」 (the must of rules/circumstance) — one “(by principle) should,” one “(by compulsion) must”; ② exception: する→「すべき」 (努力すべき, don't forget this classical change); ③ don't confuse it with the conjectural 「はず」 — べき = ought to do (duty), はず = should be (conjecture). The negative is 「べきではない」 = ought not to. To principledly assert “(by principle) one ought to …,” べきだ has the most force.

❓ FAQ

Q: 「べきだ」 and 「なければならない」 both mean ‘should do' — how do I tell them apart?

Whether it's ‘the ought of principle' or ‘the must of rules/circumstance.' 「〜べきだ」 = ought to ～ (based on ‘principle, common sense, duty,' a subjective assertion ‘this is right,' with a normative, admonishing feel): 人として、約束は守るべきだ (as a person, one ought to keep promises — appealing to principle); 上司なら、部下を守るべきだ (as a boss, one ought to protect subordinates — the ought of principle). It stresses “(by principle) it's inherently right,” a subjective value judgment. 「〜なければならない」 = must ～, have to ～ (based on ‘rule, duty, circumstance, necessity,' objectively marking ‘must do'): 明日までにレポートを出さなければならない (must submit the report by tomorrow — a rule/deadline); ビザを取らなければならない (must get a visa — a procedural necessity). It stresses “(due to some external requirement/circumstance) it won't do not to,” more objective, compulsory. Test: “(by principle/rule) one ought to …” → べきだ; “(by rule/circumstance/necessity) must …” → なければならない. E.g., 「もっと環境を守るべきだ」 = we ought to protect the environment more (appealing to principle/values); 「締め切りを守らなければならない」 = must meet the deadline (a rule's compulsion). One is “the ought of principle,” one “the must of compulsion” — different in tone and focus.

Q: For 「する」 ＋ 「べき」, why is it 「すべき」 not 「するべき」?

「すべき」 is a classical (archaic) remnant attachment. 「べき」 comes from the classical auxiliary 「べし」, which in classical grammar attaches to a verb's 終止形 (terminal form), and 「する（す）」's classical terminal form is 「す」, so with 「べき」 it's traditionally 「す＋べき＝すべき」. This classical change survives into modern usage as a fixed form: 努力すべき, 注意すべき, 対応すべき, 検討すべき. 「するべき」 isn't wrong in modern usage either (often heard colloquially), but in formal, written contexts, 「すべき」 is more standard, commoner, more tested. So: formal/written → use 「すべき」; colloquial → 「するべき」 is okay too, but 「すべき」 is always safe. Same for all “Sino-Japanese ＋ する” verbs: 勉強→勉強すべき, 準備→準備すべき, 解決→解決すべき. Memory aid: this is 「べき」's one important attachment exception — する becomes 「す」 (す＋べき), while all other verbs use the dictionary form ＋ べき directly (行くべき, 守るべき, 考えるべき). On an exam, see する ＋ べき and choose 「すべき」.

🖊️ Practice Questions (5)

Q1. 「人の悪口を言う___。」(One should not speak ill of others)

(A) べきではない　(B) はずではない　(C) なければならない　(D) べきだ

Q2. 「子供たちに正しい教育を受けさせる___。」(We should let children receive a proper education)

(A) べきだ　(B) はずだ　(C) ようだ　(D) そうだ

Q3. 「もっと早く決断す___だった。」(I should have decided sooner — regret)

(A) はず　(B) べき　(C) よう　(D) そう

Q4. 「これが私たちが守る___原則だ。」(This is the principle we should uphold — modifying a noun)

(A) べき　(B) はずの　(C) ようだ　(D) そうな

Q5. 「べきだ」と「なければならない」の違いは？

(A) 全く同じ意味で、どちらでも使える。

(B) べきだ は道義的・客観的な当然の義務；なければならない は必要性・強制の義務。

(C) べきだ は口語のみ；なければならない は書面語のみ。

(D) なければならない は禁止；べきだ は許可を表す。

Answer Explanations

1. (A) べきではない ── 「悪口を言うべきではない」(one should not speak ill) = the negative form of べき (moral duty) → べきではない.

2. (A) べきだ ── 「正しい教育を受けさせるべきだ」(should let them receive a proper education) = an objective, morally grounded judgment / duty → べきだ.

3. (B) べき ── 「もっと早く決断するべきだった」(should have decided sooner) = べきだった = "should have 〜" (regret / looking back), べき.

4. (A) べき ── 「守るべき原則」(the principle to be upheld) = べき modifying a noun (attributive form, べき + noun).

5. (B) ── べきだ = an objective moral "should" (a judgment based on common sense / reason); なければならない = necessity / obligation (must do). In nuance, べきだ has more of a "preaching" feel, while なければならない emphasizes the pressure of "having no choice."