English says must not — one crisp modal verb. Japanese has no modal verbs at all, so it builds prohibition out of pieces you already know: a te-form, the topic particle は, and a word that means "no good." Literally, 〜てはいけない says "as for doing X, it won't do." Once you see that shape, prohibition stops being a vocabulary item to memorize and becomes a small, transparent machine you can operate on any verb.
The form
Take the te-form of the verb, add は, then add a rejection word — usually いけない (plain) / いけません (polite), or ならない (formal, written), or だめ (casual):
| Verb | te-form | Prohibition |
|---|---|---|
| 吸う (to smoke) | 吸って | 吸ってはいけない |
| 入る (to enter) | 入って | 入ってはいけません |
| 撮る (to take [a photo]) | 撮って | 撮ってはならない |
ここでたばこを吸ってはいけません。
koko de tabako o sutte wa ikemasen
You must not smoke here. (polite — a rule stated to you)
芝生に入ってはいけません。
shibafu ni haitte wa ikemasen
Keep off the grass. (formal — the wording of a park sign)
ここで写真を撮ってはいけない。
koko de shashin o totte wa ikenai
You're not allowed to take photos here. (plain)
Notice the は is pronounced wa, not ha — it is the ordinary topic particle doing its ordinary job, which is the whole secret to the pattern.
The logic: topicalize the action, then reject it
Break 写真を撮ってはいけない into its literal parts. 撮って marks the action "taking a photo." は lifts that action up and makes it the topic — "as for taking a photo…". いけない (idiomatically "no good," originally the negative potential of 行く, "cannot go") then delivers the verdict: …it won't do. So the sentence is not "you must not," it is "taking-a-photo — that's no good." Japanese never orders the person around; it evaluates the action and finds it unacceptable.
This is why the choice of particle carries the entire meaning, and it makes prohibition the exact mirror image of 〜てもいい permission. Both hang off the same te-form; only the particle changes:
| Particle | Structure | Literal | Meaning |
|---|---|---|---|
| も ("even") | 撮ってもいい | "even if you take one, it's good" | You may take photos. |
| は ("as for") | 撮ってはいけない | "as for taking one, it's no good" | You must not take photos. |
ここで写真を撮ってもいいですか。
koko de shashin o totte mo ii desu ka
Is it OK to take photos here? (permission — も)
The casual contraction: 〜ちゃだめ / 〜じゃだめ
In everyday speech, ては contracts to ちゃ and では contracts to じゃ. This is the identical sound change you already met in 〜ちゃった (from 〜てしまった). Swap in だめ ("no good") and you get the sound of real spoken scolding:
| Full form | Contraction | Example |
|---|---|---|
| 〜ては (te-verbs) | 〜ちゃ | 触っては → 触っちゃ |
| 〜では (de-verbs) | 〜じゃ | 遊んでは → 遊んじゃ |
嘘をついちゃだめだよ。
uso o tsuicha dame da yo
You mustn't tell lies. (informal — gently telling off a child or friend)
遊んじゃだめ。まず宿題やって。
asonja dame. mazu shukudai yatte
No playing — do your homework first. (informal — a parent)
触っちゃだめ！熱いよ。
sawatcha dame! atsui yo
Don't touch it! It's hot. (informal — an urgent warning)
Whether a verb contracts to ちゃ or じゃ depends only on whether its te-form ends in て or で — the same split that governs the te-form itself. 触る → 触って → 触っちゃ; 遊ぶ → 遊んで → 遊んじゃ; 飲む → 飲んで → 飲んじゃ.
お酒を飲んじゃだめだよ、運転するんだから。
osake o nonja dame da yo, unten suru n da kara
You mustn't drink — you're driving. (informal)
Same grammar, very different social force
Here is a distinction that textbooks often blur. 〜てはいけません and 〜ちゃだめ are grammatically the same construction, but they occupy opposite ends of the social scale:
- 〜てはいけない / 〜てはいけません (plain / polite) — states an external rule: a law, a sign, a policy. It is impersonal. This is the voice of the museum, the landlord, the school regulation.
- 〜てはならない (formal, written / literary) — the strongest and stiffest, found in laws, contracts, and moral pronouncements ("one must not…").
- 〜ちゃだめ / 〜ちゃいけない (informal) — the intimate, spoken scolding of parents, partners, and close friends. だめ especially is warm and personal, not the language of authority but of someone who cares.
So the same prohibition can be delivered as cold regulation or as affectionate telling-off purely by the register you choose.
Softer than a rule: 〜ないでください
If you only want to ask someone not to do something — a polite request, not a hard prohibition — reach instead for 〜ないでください ("please don't…"). It requests; 〜てはいけない forbids.
ここに座らないでください。
koko ni suwaranai de kudasai
Please don't sit here. (a polite request — softer than a ban)
心配しないで。大丈夫だから。
shinpai shinai de. daijōbu da kara
Don't worry — it'll be fine. (informal, caring)
The difference is force: 入ってはいけません is "entry is forbidden"; 入らないでください is "please don't go in." A guard uses the first; a host who'd rather you didn't wander into the kitchen uses the second.
Common mistakes
❌ 今日は来てはいけないよ。
kyō wa kite wa ikenai yo
Incorrect if you meant 'you don't have to come' — this actually says 'you must NOT come.'
✅ 今日は来なくてもいいよ。
kyō wa konakute mo ii yo
You don't have to come today.
The single most damaging error for English speakers: confusing must not (〜てはいけない) with don't have to (〜なくてもいい). English keeps these worlds apart, but a learner reaching for "you don't need to" often lands on the prohibition and forbids exactly what they meant to excuse.
❌ 飲んちゃだめ。
noncha dame
Incorrect — 飲む takes the で-form (飲んで), so it contracts to じゃ, not ちゃ.
✅ 飲んじゃだめ。
nonja dame
You mustn't drink it.
❌ ここで写真を撮るはいけない。
koko de shashin o toru wa ikenai
Incorrect — は attaches to the te-form (撮って), never to the dictionary form (撮る).
✅ ここで写真を撮ってはいけない。
koko de shashin o totte wa ikenai
You can't take photos here.
❌ 芝生に入っちゃだめ。（立て札）
shibafu ni haiccha dame (tatefuda)
Too casual for a public sign — ちゃだめ is intimate scolding, not official language.
✅ 芝生に入ってはいけません。
shibafu ni haitte wa ikemasen
Keep off the grass. (the register a sign would actually use)
Key takeaways
- te-form + は + いけない / だめ = prohibition. Literally "as for doing X, it's no good" — Japanese rejects the action, it doesn't command the person.
- It is the mirror of 〜てもいい permission: same te-form, particle は (forbid) vs も (permit).
- ては → ちゃ, では → じゃ in casual speech — the same contraction as 〜ちゃった.
- Register is everything: 〜てはいけません = an external rule; 〜ちゃだめ = an intimate scolding; 〜てはならない = formal/legal.
- To merely ask someone not to, use the softer 〜ないでください, not the outright ban.
- Never confuse prohibition (〜てはいけない, "must not") with 〜なくてもいい ("don't have to") — see the obligation & permission overview for the full map.
Now practice Japanese
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Start learning Japanese→Related Topics
- 〜てもいい: Permission ('may')N4 — How 〜てもいい grants and asks permission — literally 'even if you do it, it's fine' — the polite variants 〜てもいいでしょうか and 〜てもかまいません, and the も/は symmetry with prohibition.
- 〜なくてもいい: No Need ToN4 — How 〜なくてもいい grants permission NOT to do something — the exact cancellation of obligation — and why English speakers must keep it clear of 〜てはいけない ('must not').
- Obligation Forms ComparedN3 — A decision guide to the whole 'must / have to / should / forced to / need not' family in Japanese — なければならない, ないといけない, なきゃ, べき, ざるを得ない, なくてもいい — sorted by register and nuance.
- 〜なければならない: Obligation ('must')N4 — The core Japanese way to say something must be done — a double negative meaning 'if you don't do it, it won't do' — plus how to build it correctly from the ない-stem and how ならない, いけない, and ねばならない differ.
- The て-form: Japanese's Universal ConnectorN4 — Why the tenseless, politeness-free て-form is the single most productive conjugation in Japanese — the hinge that feeds requests, progressives, sequence, permission, and dozens more constructions.