Must, must not, may and should

These patterns are made of two ideas you already have: the negative stem なけれ and the conditional ば / ては. Understand the logic and you can build any of them yourself.

て-form + もいい — "even if you do it, it's fine" — gives permission.

Without a question mark and with a rising tone, てもいいですか is how you ask permission. The answer is いいですよ (sure) or ちょっと… (that's a problem).

て-form + はいけない — "if you do it, it won't do" — forbids.

In casual speech, 〜ちゃだめ / 〜じゃだめ is more common (食べちゃだめ: don't eat it!).

ない-form without い + ければならない — "if you don't do it, it won't do" — expresses obligation.

The logic: 勉強しない (don't study) + ければ (if) + ならない (it won't do) = if you don't study, it won't do. So you must.

There are several interchangeable forms:

In speech, the end of the sentence is often dropped. You'll hear 行かなきゃ… and 行かないと… and understand them from context. Both mean "I have to go."

ない-form without い + くてもいい — "even if you don't do it, it's fine".

Compare it carefully with the "must" family:

Past form + ほうがいい gives advice: "it's better to...". The verb is in the past form even though the advice is about the future.

Another word for "should" is べき: dictionary form + べきだ. It is firmer: a moral "ought".

Beyond the verb patterns, 必要がある (there is a need) and 〜が必要だ express necessity as a noun.

Rules, permission and obligation appear together in notices and instructions.

• てもいい: may. てはいけない: must not.
• なければならない / なくてはいけない / なきゃ: must.
• なくてもいい: don't have to.
• ほうがいい: should, advisable (past form for positive, ない-form for negative).
• べき: ought to.