Translation guide
The English modal verb 'must' expresses obligation, necessity, or strong inference. Japanese uses various grammatical patterns depending on the type of obligation (personal, social, rule-based), negation (must not vs. don't have to), and level of formality.
Expressing that someone has to do something, either due to rules, duty, or personal feeling.
Standard pattern for 'must' or 'have to'. Used for both personal and external obligations. Neutral formality.
宿題をしなければならない。
I must do my homework.
明日早く起きなければならない。
I have to get up early tomorrow.
Very similar to ～なければならない, but slightly more subjective or personal. Often used in spoken Japanese.
もう行かなければいけない。
I must go now.
Casual spoken form. Often used when the obligation is felt personally or situationally.
そろそろ帰らないといけない。
I have to go home soon.
Formal/written equivalent of ～なければならない. Often used in official contexts.
規則を守らなくてはならない。
We must follow the rules.
Expresses 'should' or 'ought to', implying moral obligation or strong advice. Not as forceful as 'must' but often used for recommendations.
もっと勉強するべきだ。
You should study more.
Expressing that something is forbidden or not allowed.
Standard pattern for 'must not' or 'may not'. Used in rules and direct prohibitions.
ここで写真を撮ってはいけない。
You must not take photos here.
Formal/written prohibition. Stronger and more official than ～てはいけない.
この情報を外部に漏らしてはならない。
You must not leak this information outside.
Casual spoken prohibition. ちゃ is contraction of ては, じゃ is contraction of では.
触っちゃだめ！
Don't touch! (You must not touch!)
Expressing a logical conclusion or strong belief about a situation.
Expresses strong conviction 'must be', 'no doubt'. Used for logical deductions.
彼はきっと成功するに違いない。
He must surely succeed.
Indicates expectation based on reason or knowledge: 'should be', 'must be'. Slightly less forceful than ～に違いない.
彼はもう着いているはずだ。
He must have arrived already.
Expresses probability or conjecture. でしょう is polite. Often used for 'must be' in everyday speech.
疲れたでしょう。
You must be tired.
Expressing that something is not required. Note: English 'must not' vs. 'don't have to' are distinct; Japanese patterns differ.
Standard way to say 'don't have to' or 'need not'. Casual to polite depending on ending.
明日は来なくてもいいです。
You don't have to come tomorrow.
More formal/literal 'there is no need to'. Often used in written instructions.
急ぐ必要はない。
You don't need to hurry.
In English, 'must not' is prohibition, while 'don't have to' is lack of obligation. Japanese uses completely different patterns: ～てはいけない (must not) vs. ～なくてもいい (don't have to). Mixing them up will change the meaning entirely.
～なければならない implies an unavoidable obligation (must/have to), while ～べきだ is a moral or logical 'should'. Use ～べきだ for advice, and ～なければならない for rules or necessities.
税金を払わなければならない。
You must pay taxes.
もっと野菜を食べるべきだ。
You should eat more vegetables.
明日までにこのレポートを終えなければならない。
I must finish this report by tomorrow.
冗談でしょう！
You must be joking!