〜てはいけない: Saying 'Must Not' (Prohibition in Japanese)
The stance it takes: a flat ban
〜てはいけない is a prohibition — the speaker declares an action off-limits. It's the everyday way to say "must not" or "you may not," the voice of signs, teachers, and parents laying down a rule. The stance is absolute rather than a suggestion: it doesn't ask you to reconsider, it forbids. That makes it the direct flip side of permission — where 〜てもいい grants "you may," 〜てはいけない withholds it with "you may not."
ここでたばこを吸ってはいけません。
ここで たばこを すっては いけません。
You must not smoke here.
先生：授業中に寝てはいけませんよ。
せんせい：じゅぎょうちゅうに ねては いけませんよ。
Teacher: You must not sleep during class.
ヤッタンの弟、その箱を開けてはいけない。
ヤッタンの おとうと、その はこを あけては いけない。
Yattan's little brother, you must not open that box.
What it's built on
The prohibition rides on the verb's て-form: take it, then add はいけない (plain) or はいけません (polite). The casual contraction squeezes ては down to ちゃ, and では down to じゃ:
| Verb | て-form | Plain | Polite | Casual |
|---|---|---|---|---|
| 吸う (smoke) | 吸って | 吸ってはいけない | 吸ってはいけません | 吸っちゃいけない |
| 食べる (eat) | 食べて | 食べてはいけない | 食べてはいけません | 食べちゃいけない |
| 飲む (drink) | 飲んで | 飲んではいけない | 飲んではいけません | 飲んじゃいけない |
| 遊ぶ (play) | 遊んで | 遊んではいけない | 遊んではいけません | 遊んじゃいけない |
Notice the で-ending て-forms (飲んで, 遊んで) contract to じゃ, not ちゃ: 飲んじゃいけない.
How strong it sounds: from casual だめ to formal ならない
The ban itself stays the same; how heavy it lands depends on which ending you attach. In conversation, full 〜てはいけない sounds stiff — friends and family reach for the shortened ちゃ / じゃ, often dropping いけない to a blunt だめ:
モチ、ここに入っちゃだめだよ。
モチ、ここに はいっちゃ だめだよ。
Mochi, you can't go in here.
入っては → 入っちゃ; いけない → だめ for a casual feel.
まだ帰っちゃいけないよ。
まだ かえっちゃ いけないよ。
You can't leave yet.
この水を飲んじゃだめ。
この みずを のんじゃ だめ。
Don't drink this water.
飲んでは → 飲んじゃ because the て-form ends in で.
At the other end of the scale, the いけない slot swaps out for a heavier word. This is the strength ladder — same prohibition, rising formality:
| Form | Core idea | Example |
|---|---|---|
| 〜てはいけない | standard 'must not' (neutral) | 触ってはいけない = must not touch |
| 〜てはだめ | casual, spoken | 触っちゃだめ = you can't touch that |
| 〜てはならない | formal / written, rules & laws | 触れてはならない = touching is prohibited |
You'll meet 〜てはならない in written rules, contracts, and signs. It carries the same "must not" meaning but feels weightier and more official than 〜てはいけない.
Set against permission and obligation
The real difficulty isn't the strength ladder — it's that three neighbouring patterns hang off similar shapes and all get glossed with English "must." Here's how the prohibition lines up against them:
- 〜てはいけない = must not do (prohibition): 食べてはいけない ("you must not eat it").
- 〜てもいい = may do (permission): 食べてもいい ("you may eat it").
- 〜なくてもいい = don't have to do (no obligation): 食べなくてもいい ("you don't have to eat it").
- 〜なければならない = must do (obligation): 食べなければならない ("you must eat it").
So 〜てはいけない bans an action, while 〜なければならない demands one. If you instead want to politely ask someone not to do something (a gentle request rather than a hard rule), use 〜ないでください.
The traps to watch
- Forgetting the て-form. It builds on the て-form, not the dictionary form: 行ってはいけない (✓), not 行くはいけない (✗).
- Using ちゃ for で-ending verbs. 飲んで, 遊んで, 死んで take じゃ: 飲んじゃいけない, not 飲んちゃいけない.
- Mixing up ban and obligation. 〜てはいけない = must NOT; 〜なければならない = must DO. They are opposites, not the same "must."
- Confusing it with "don't have to." 食べてはいけない ("must not eat") is very different from 食べなくてもいい ("don't have to eat").
The short version
- て-form + はいけない/はいけません = "must not / not allowed to."
- Casual contractions: 〜ちゃいけない, and 〜じゃいけない for で-ending verbs.
- Swap いけない for だめ (casual) or ならない (formal/written).
- Opposite of 〜てもいい (permission); contrast with 〜なければならない (must do).
Practice it
Choose the correct 〜てはいけない form (polite, casual ちゃ/じゃ, or variant).
＿＿＿に入れるのに最もよいものを選びなさい。
Choose the grammar pattern that best completes the sentence.
ここでたばこを＿＿＿。
Frequently asked questions
What does 〜てはいけない mean?
It means 'must not' or 'not allowed to' — it states a prohibition. ここで吸ってはいけません = 'You must not smoke here.' Build it from the verb's て-form plus はいけない (plain) or はいけません (polite).
What's the casual form of 〜てはいけない?
ては contracts to ちゃ and では contracts to じゃ: 食べてはいけない → 食べちゃいけない, 飲んではいけない → 飲んじゃいけない. You can also shorten いけない to だめ: 食べちゃだめ.
When do I use じゃ instead of ちゃ?
Use じゃ when the て-form ends in で — that is, with verbs like 飲む (飲んで), 遊ぶ (遊んで), 読む (読んで). So 飲んではいけない becomes 飲んじゃいけない.
How is 〜てはいけない different from 〜なければならない?
They're opposites. 〜てはいけない means 'must NOT do' (a ban), while 〜なければならない means 'must DO' (an obligation). 行ってはいけない = 'you must not go'; 行かなければならない = 'you must go.'
Where 〜てはいけない fits
Shaky on this? Get these solid first
- 〜てください · N5 — 〜てください is the standard way to politely ask someone to do something ('please do…'). Learn the formation, the negative 〜ないでください, softer variations, and see clear examples.
- The て-form · N5 — The て-form is the gateway to dozens of Japanese grammar patterns. Learn the conjugation rules for every verb group, the famous song trick, and see clear examples.
Don't confuse it with
- 〜てもいい · N5 — 〜てもいい means 'it's okay to' or 'may': verb て-form + もいい(です). Learn to ask permission with 〜てもいいですか, reply naturally, and use 〜てもかまいません.
- 〜ないでください · N5 — 〜ないでください is the polite way to ask someone not to do something ('please don't…'). Learn the formation from the ない-form, with examples and how it pairs with 〜てください.
More N5 grammar to explore
Keep going with related patterns at the same level:
- いちばん — いちばん (一番) makes superlatives in Japanese: just put it before an adjective to say 'the most ~' or 'the ~est.' Learn the で / の中で scope, how to ask and answer, and the common より mix-up.
- 〜すぎる — 〜すぎる means 'too much' or 'excessively': 食べすぎる (eat too much), 高すぎる (too expensive). Learn the verb, い-adjective, and な-adjective formations, plus the irregulars よすぎる and なさすぎる.
- 〜より / 〜のほうが — より means 'than' and のほうが marks the bigger side: BよりAのほうが高い = 'A is taller than B.' Learn both frames, how to answer どちらが questions, and word order.
- 〜でしょう / 〜だろう — でしょう means 'probably' and, with a rising tone, 'right?'. Learn how it attaches to verbs, adjectives, and nouns, its casual form だろう, and why forecasts love it.
- 〜けど / 〜が — けど and が join two clauses to mean 'but' or 'although': 高いけど買う. が is more formal, けど is conversational. Learn both, plus the polite softener use (すみませんが…).
- 〜がほしい — ほしい expresses wanting a thing (a noun): 〜がほしい. Learn the formation, the が particle, the 〜てほしい variant ('want someone to do'), and how it differs from 〜たい.