Meaning & Usage
てはいけない (te wa ikenai) means "must not" or "may not." Use it to forbid an action — setting a rule, issuing a warning, or stating a social expectation.
The pattern signals that an action is not acceptable or not permitted. It's stronger than advice — closer in weight to a rule or moral prohibition. English parallels: "You must not run in the hallways" or "Smoking is not allowed here."
Social register matters here. Use the plain form てはいけない in casual speech — a parent talking to a child, close friends. Switch to the polite form てはいけません in formal settings: school, work, public announcements, or any conversation with someone of higher status. Getting this wrong sounds rude or childish.
Despite Japanese being indirect in many social contexts, てはいけない is blunt. There's no ambiguity — the action is simply not allowed. That directness is why it appears on public signs, in instructions from authority figures, and in parenting. Between adult peers, Japanese speakers often soften prohibitions with gentler patterns (see Related Grammar below), but when a rule needs stating clearly, this is the go-to.
Picture the universal red circle-slash "no" sign. Anywhere you see "No Entry," "No Smoking," or "Do Not Touch" — てはいけない is the Japanese behind it.
Structure & Formation
Take the て-form of a verb and attach はいけない or its polite equivalent はいけません.
| Form | Pattern | Register |
|---|---|---|
| Plain (casual) | Verb て-form + はいけない | Informal |
| Polite | Verb て-form + はいけません | Formal |
| Casual contraction | Verb て-form (contracted) + はだめ | Very casual |
The particle は (wa) functions as a topic/contrast marker, adding quiet emphasis: "As for doing X — that is not acceptable." The verb いける means "to be acceptable," making いけない literally "it is not acceptable." The logic tracks cleanly: て-form + は + いけない = "doing [verb] is not acceptable."
In spoken Japanese, ては contracts to ちゃ (when the て-form ends in て) or じゃ (when it ends in で). So 行ってはいけない becomes 行っちゃいけない, and 飲んではいけない becomes 飲んじゃいけない. These contractions are everywhere in natural speech.
- Godan verbs: 飲む → 飲んではいけない
- Ichidan verbs: 食べる → 食べてはいけない
- Irregular verb する: する → してはいけない
- Irregular verb くる: くる → きてはいけない
Example Sentences
Rules and Signs
ここで煙草を吸ってはいけません。
Koko de tabako wo sutte wa ikemasen.
You must not smoke here.
この部屋に入ってはいけない。
Kono heya ni haitte wa ikenai.
You must not enter this room.
ここに車を止めてはいけません。
Koko ni kuruma wo tomete wa ikemasen.
You may not park your car here.
School and Classroom Rules
授業中に携帯を使ってはいけない。
Jugyouchuu ni keitai wo tsukatte wa ikenai.
You must not use your phone during class.
図書館で話してはいけません。
Toshokan de hanashite wa ikemasen.
You must not talk in the library.
試験中にカンニングしてはいけません。
Shiken chuu ni kanningu shite wa ikemasen.
You must not cheat during exams.
Safety and Health
この水を飲んではいけません。
Kono mizu wo nonde wa ikemasen.
You must not drink this water.
この薬を子供に飲ませてはいけません。
Kono kusuri wo kodomo ni nomasete wa ikemasen.
You must not give this medicine to children.
赤信号で渡ってはいけない。
Akashingou de watatte wa ikenai.
You must not cross on a red light.
Moral and Social Rules
嘘をついてはいけません。
Uso wo tsuite wa ikemasen.
You must not tell lies.
人の物を取ってはいけない。
Hito no mono wo totte wa ikenai.
You must not take other people's things.
夜遅く騒いではいけません。
Yoru osoku sawaide wa ikemasen.
You must not make noise late at night.
Parenting and Daily Life
そんなに速く走ってはいけないよ。
Sonna ni hayaku hashitte wa ikenai yo.
You must not run so fast, okay?
知らない人についていってはいけません。
Shiranai hito ni tsuite itte wa ikemasen.
You must not follow strangers.
Common Mistakes
Mistake 1: Using the Dictionary Form Instead of the て-form
❌ ここで走るはいけない。
✅ ここで走ってはいけない。
The pattern requires the て-form, not the dictionary form. The て-form of 走る is 走って. Conjugate the verb before adding はいけない — every time, no exceptions.
Mistake 2: Confusing てはいけない with てはいけません in Register
❌ 先生、ここで話してはいけない？ (Too casual for a teacher)
✅ 先生、ここで話してはいけませんか？
Using the plain form いけない with a teacher, boss, or senior sounds rude. Always use いけません in formal or polite contexts. Japanese social hierarchy makes this a bigger deal than it might seem to English speakers.
Mistake 3: Forgetting the Particle は
❌ ここで食べていけない。
✅ ここで食べてはいけない。
The particle は is not optional — it's part of the pattern. Dropping it produces unnatural Japanese. Note: ていけない (without は) does exist but leans toward expressing impossibility rather than prohibition, and is not used this way at N5 level.
Mistake 4: Using てはいけない for Personal Self-Restraint
❌ 私はケーキを食べてはいけない。(Implies someone else is forbidding you)
✅ 私はケーキを食べてはいけないと思っている。(More natural with added context)
てはいけない implies an external rule or authority. Said about yourself without context, it sounds like someone else is doing the forbidding — not that you're choosing restraint. Adding 思っている or a reason makes self-directed uses land naturally.
Mistake 5: Mixing Up てはいけない and なければならない
❌ 宿題をしてはいけない。(This says "You must NOT do homework" — the opposite!)
✅ 宿題をしなければならない。(You must do homework.)
てはいけない = prohibition (must NOT do). なければならない = obligation (must DO). Swapping them flips the meaning of the sentence entirely. Quick check: いけない = it's not okay; ならない = it can't be avoided.
Cultural Notes
Walk through any Japanese train station, park, temple, or school and you'll find てはいけません on signs everywhere. More formal written rules often use てはなりません — a stiffer, literary equivalent. Japan's emphasis on group harmony means prohibitions are stated plainly and generally taken seriously.
Between parent and child, てはいけない is firm but matter-of-fact — not harsh, just direct. Between adults, the same pattern can feel heavy. In those contexts, ないほうがいい (it would be better not to) or a plain やめて (please stop) are softer alternatives that feel less like a reprimand.
The contractions ちゃいけない and じゃいけない are so common in spoken Japanese that many learners hear them before encountering the full form. If you've watched anime or Japanese dramas, you've almost certainly heard them already. Knowing both forms is essential for understanding natural conversation.
Related Grammar Points
- Te-Form Connector: Linking Actions, States, and Reasons (Grammar N5)
- ない — Negative Form (Not) (Grammar N5)
- てください — Please Do (Grammar N5)
- ている — Progressive and Resultant State (Grammar N5)
- てもいい — May, It's Ok To (Grammar N5)
- あげる — To Give (Giving to Others) (Grammar N5)
JLPT Tips
On the N5 exam, てはいけない appears in two ways: grammar recognition (identifying that the pattern expresses prohibition) and sentence completion (choosing the correct verb form to fill a gap).
The key skill is て-form conjugation. If the exam gives you a plain-form verb and asks you to build a prohibition, convert it to the て-form first. Practice all three verb groups — godan, ichidan, and the irregulars する and くる — until the conjugations are automatic.
Watch for distractor answers in multiple-choice questions. Common traps include: the dictionary form instead of the て-form, てもいい (permission — the opposite meaning), and なければならない (obligation). Knowing what each related pattern means lets you eliminate wrong answers without hesitating.
Politeness level clues in the question stem can also guide you. Formal setting (school, hospital, official announcement) → てはいけません. Casual setting (family, close friends) → てはいけない. This distinction alone can tip borderline questions in your favor.