Obligation & Prohibition
Learn how to express obligation, prohibition, and lack of necessity in Japanese.
Obligation & Prohibition
Japanese expresses 'must,' 'must not,' and 'don't have to' through several patterns built on the negative form of verbs. The core logic is consistent: obligation uses a double negative ('if you don't do it, it won't do'), while prohibition attaches a negative judgment to the te-form.
These patterns come in multiple formality levels. Formal speech uses なければなりません, everyday polite speech uses なくてはいけません, and casual speech contracts these into なきゃ and なくちゃ. Choosing the right level matters — using the full formal pattern in casual conversation sounds stiff, while using contractions in a business meeting sounds too casual.
This page also covers べき (should/ought to), which expresses moral obligation or strong recommendations rather than necessity.
The most formal 'must' pattern. Take the nai-form of a verb, replace ない with なければならない.
[nai-stem] + なければならない / なりません
- 行く → 行か（ない）→ 行かなければならない
- 食べる → 食べ（ない）→ 食べなければならない
- する → し（ない）→ しなければならない
The literal meaning is 'if [one] doesn't do it, it won't become [acceptable].' ならない can be replaced with なりません for polite speech.
For past obligation, change ならない to ならなかった: 行かなければならなかった (had to go).
Examples
- I must take medicine every day.
毎日、薬を飲まなければなりません。
- I have to submit the report by tomorrow.
明日までにレポートを出さなければならない。
- In Japan, you must take off your shoes.
日本では靴を脱がなければなりません。
- I had to wake up earlier.
もっと早く起きなければならなかった。
A slightly less formal 'must' pattern, common in everyday polite speech. Take the nai-form, replace ない with なくてはいけない.
[nai-stem] + なくてはいけない / いけません
- 行く → 行かなくてはいけない
- 食べる → 食べなくてはいけない
The literal meaning is 'if [one] doesn't do it, it's not OK.' いけない can be replaced with いけません for polite speech, or だめ for a more colloquial tone.
なければならない and なくてはいけない are largely interchangeable. なければならない sounds slightly more formal and is more common in writing; なくてはいけない is more conversational.
Examples
- I have to do my homework.
宿題をしなくてはいけません。
- I have to bring my passport.
パスポートを持っていかなくてはいけない。
- You need to eat your vegetables too.
野菜も食べなくてはいけませんよ。
- I have to head home soon.
そろそろ帰らなくてはいけない。
In casual speech, the obligation patterns contract dramatically:
- なければ → なきゃ
- なくては → なくちゃ
The いけない / ならない ending is often dropped entirely — the contraction alone implies 'must.'
- 行かなきゃ (gotta go)
- 食べなくちゃ (gotta eat)
You can also keep a short ending: なきゃいけない, なくちゃだめ.
These contractions are extremely common in spoken Japanese. Even in anime and dramas, you'll hear なきゃ and なくちゃ far more often than the full forms.
Examples
- I gotta go now.
もう行かなきゃ。
- I need to go to bed early.
早く寝なきゃいけない。
- I have to finish this by tomorrow.
明日までに終わらせなくちゃ。
- You gotta eat properly.
ちゃんと食べなくちゃだめだよ。
To express prohibition ('must not,' 'not allowed to'), attach てはいけない to the te-form of a verb.
[te-form] + は + いけない / いけません
- 撮る → 撮って → 撮ってはいけない
- 使う → 使って → 使ってはいけない
The literal meaning is 'if [you] do it, it's not OK.' いけません is the polite form.
This pattern is used for rules, regulations, and authoritative prohibitions — a teacher to students, a sign in a museum, a parent to a child.
Examples
- You must not take photos here.
ここで写真を撮ってはいけません。
- You must not use your phone during class.
授業中にスマホを使ってはいけない。
- You must not press this button.
このボタンを押してはいけません。
- You can't eat yet.
まだ食べてはいけないよ。
In casual speech, てはいけない contracts to ちゃだめ (or じゃだめ for で-ending te-forms):
- ては → ちゃ
- では → じゃ
[te-form contraction] + だめ
- 撮っちゃだめ (can't take photos)
- 走っちゃだめ (can't run)
- 飲んじゃだめ (can't drink)
だめ literally means 'no good.' This pattern is very common between friends, parents and children, and in informal settings. Adding だよ softens it slightly.
Examples
- Don't run here!
ここで走っちゃだめ！
- You shouldn't lie.
嘘をついちゃだめだよ。
- You shouldn't stay up late.
夜遅くまで起きてちゃだめだよ。
- Don't take other people's things.
人のものを取っちゃだめ。
To express lack of necessity ('don't have to,' 'it's OK not to'), take the nai-form and replace ない with なくてもいい.
[nai-stem] + なくてもいい / いいです
- 行く → 行かなくてもいい (don't have to go)
- 食べる → 食べなくてもいい (don't have to eat)
- する → しなくてもいい (don't have to do)
The literal meaning is 'even if [you] don't do it, it's fine.' This is the opposite of obligation — it releases someone from a requirement.
Don't confuse this with prohibition: なくてもいい means the action is optional, while てはいけない means the action is forbidden.
Examples
- You don't have to come tomorrow.
明日は来なくてもいいです。
- You don't have to eat all of it.
全部食べなくてもいい。
- You don't have to push yourself.
無理しなくてもいいですよ。
- There's no rush, so take your time.
急がなくてもいいから、ゆっくりやって。
べき expresses moral obligation, strong advice, or what is expected. It attaches to the dictionary form of verbs.
[dictionary form] + べき / べきだ / べきです
- 勉強するべき (should study)
- 守るべき (should keep/protect)
- 言うべき (should say)
Exception: する can become either するべき or すべき (the more traditional form).
For past regret ('should have'), use べきだった: 言うべきだった (should have said it).
べき is stronger than たほうがいい (had better) — it implies a sense of duty or principle, not just practical advice.
Examples
- Students should study every day.
学生は毎日勉強するべきです。
- You should keep your promises.
約束は守るべきだ。
- I should have said something sooner.
もっと早く言うべきだった。
- You should exercise for your health.
健康のために運動するべきです。