Grammar

Must & Should in Japanese: なければならない, べき, ほうがいい

義N4–N3

Three core expressions cover obligation and advice in Japanese: なければならない (strong obligation — must), べき (moral duty — ought to), and ほうがいい (advice — you'd better / it's better to). Each has distinct nuance and register.

〜なければならない — strong obligation (must)

〜なければならない means "must do" or "have to do". It expresses an external obligation or rule. In casual speech it contracts to 〜なきゃ or 〜なくちゃ. 〜なければいけない is interchangeable (slightly more conversational).

明日までにレポートを出さなければならない。あしたまでにレポートをださなければならない。I must submit the report by tomorrow.

毎日薬を飲まなきゃ。まいにちくすりをのまなきゃ。I have to take medicine every day. (casual contraction)

パスポートを持ってこなければなりません。パスポートをもってこなければなりません。You must bring your passport. (formal)

〜べき — moral obligation / ought to

〜べき (or べきだ) attaches to the dictionary form of a verb and expresses moral duty, strong advice, or what is considered the right thing to do. It is stronger and more prescriptive than ほうがいい. In formal writing, 〜べきである is used.

約束は守るべきだ。やくそくはまもるべきだ。One ought to keep their promises.

もっと早く謝るべきだった。もっとはやくあやまるべきだった。You should have apologized sooner. (past regret)

このことは上司に報告すべきです。このことはじょうしにほうこくすべきです。You should report this to your supervisor.

〜ほうがいい — advice / recommendation

〜ほうがいい means "it's better to" or "you'd better". It gives softer advice or recommendations. Past tense (〜たほうがいい) is actually the standard form for positive advice. The negative form 〜ないほうがいい advises against an action.

早く寝たほうがいいよ。はやくねたほうがいいよ。You'd better go to bed early.

無理しないほうがいい。むりしないほうがいい。You'd better not push yourself too hard.

一度医者に行ったほうがいいと思います。いちどいしゃにいったほうがいいとおもいます。I think you should see a doctor once.

〜なくてもいい — no need to (absence of obligation)

〜なくてもいい means "you don't have to" — it negates obligation without implying prohibition. It is the opposite of 〜なければならない on the obligation scale.

今日は来なくてもいいですよ。きょうはこなくてもいいですよ。You don't have to come today.

全部食べなくていいよ。ぜんぶたべなくていいよ。You don't have to eat everything. (casual: なくていい)

Common mistakes

Wrong

Using べき for soft personal advice to a friend

Right

Use ほうがいい for advice; reserve べき for moral/professional obligation

「もっと運動するべきだ」to a friend sounds preachy and judgmental. 「もっと運動したほうがいいよ」is natural friendly advice. べき implies a moral standard the person is violating.

Wrong

するべき vs すべき — thinking they differ in meaning

Right

Both are correct; すべき is more formal/literary

する + べき can contract to すべき. The meaning is identical. すべきではない = するべきではない. In modern speech, するべき is more common; in formal writing, すべき.

Wrong

たほうがいい vs ほうがいい for positive advice

Right

Use past tense (〜たほうがいい) for positive advice, not plain present

While both exist, 〜たほうがいい (past form) is the standard and natural form for "you should do X". 〜するほうがいい sounds slightly unnatural in many contexts.

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Frequently asked questions

What is the difference between べき and ほうがいい?

べき expresses moral obligation or duty — what someone "ought to" do as a matter of principle. ほうがいい gives practical advice — "it's better to". べき is stronger and can sound judgmental; ほうがいい is softer and more like a suggestion. Use ほうがいい when advising friends; べき for moral or professional statements.

How do you say "I don't have to" in Japanese?

〜なくてもいい (or 〜なくていい in casual speech). This negates obligation: 今日は行かなくてもいい = "I don't have to go today". Be careful: 〜てはいけない means "must NOT" (prohibition), which is very different from 〜なくてもいい (no need to).

What is the casual contraction of なければならない?

The most common contractions: なければ → なきゃ (very common in speech). なければならない → なきゃ (dropping ならない entirely in very casual speech). Also: なければならない → なくてはならない → なくちゃ. Example: 行かなきゃ = I've got to go.