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.
I must do my homework.
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.
I must go now.
Casual spoken form. Often used when the obligation is felt personally or situationally.
I have to go home soon.
Formal/written equivalent of ～なければならない. Often used in official contexts.
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.
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.
Expressing a logical conclusion or strong belief about a situation.
Expresses strong conviction 'must be', 'no doubt'. Used for logical deductions.
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.
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.
I must finish this report by tomorrow.
You must not take photos here.
Formal/written prohibition. Stronger and more official than ～てはいけない.
You must not leak this information outside.
Casual spoken prohibition. ちゃ is contraction of ては, じゃ is contraction of では.
Don't touch! (You must not touch!)
He must surely succeed.
Indicates expectation based on reason or knowledge: 'should be', 'must be'. Slightly less forceful than ～に違いない.
He must have arrived already.
Expresses probability or conjecture. でしょう is polite. Often used for 'must be' in everyday speech.
You must be tired.
You don't have to come tomorrow.
More formal/literal 'there is no need to'. Often used in written instructions.
You don't need to hurry.
You must not go.
～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.
You must pay taxes.
You should eat more vegetables.
You must be joking!