てはいけない: How to Say "Must Not" in Japanese

What Does てはいけない Mean?

てはいけない (te wa ikenai) expresses prohibition — it marks an action as not allowed or unacceptable. The closest English equivalent is "must not" or "you cannot (do that)."

The key word is いけない (ikenai), which comes from 行けない (ikenai), the negative potential form of 行く (iku, "to go"). That origin gives it the sense of "cannot proceed / not acceptable." Attach it to the て-form of any verb and you have a complete, grammatical prohibition.

Japanese uses two formality levels for this construction:

• Plain: 〜てはいけない (te wa ikenai) — everyday speech, general rules
• Polite: 〜てはいけません (te wa ikemasen) — formal situations, public notices, talking to strangers or superiors

Both are grammatically correct; your listener and setting determine which to use.

You already know how to use てください for polite requests — てはいけない takes that same te-form foundation and turns a request into a firm rule. Where てください says please do, てはいけない says must not do.

How to Form てはいけない

Structure: [Verb て-form] + はいけない

The te-form is the only moving part. Japanese verbs split into three groups, each with slightly different te-form rules:

Group 1 (U-verbs) — the ending changes before て: - く → いて: 書く (kaku) → 書いて (kaite) - ぐ → いで: 泳ぐ (oyogu) → 泳いで (oyoide) - む / ぶ / ぬ → んで: 飲む (nomu) → 飲んで (nonde) - う / つ / る → って: 使う (tsukau) → 使って (tsukatte) - す → して: 話す (hanasu) → 話して (hanashite) - ※ Exception: 行く (iku) → 行って (itte) — not 行いて

Group 2 (RU-verbs) — drop る, add て: - 食べる (taberu) → 食べて (tabete) - 見る (miru) → 見て (mite)

Irregular verbs: - する (suru) → して (shite) - くる (kuru) → きて (kite)

Once you have the て-form, はいけない follows immediately. For the polite form, swap いけない for いけません — everything before it stays the same.

Core Prohibition Forms Reference

てはいけない in Real Situations

Five grounded examples — from offices, schools, public signs, and home — show how naturally this pattern fits daily life:

1. Office rule 会議中に携帯を使ってはいけません。 Kaigi-chū ni keitai o tsukatte wa ikemasen. You must not use your phone during meetings.

2. School corridor 廊下を走ってはいけない。 Rōka o hashitte wa ikenai. You must not run in the hallway.

3. Tourist site ここで写真を撮ってはいけません。 Koko de shashin o totte wa ikemasen. You must not take photos here.

4. Parent to child 子どもはタバコを吸ってはいけない。 Kodomo wa tabako o sutte wa ikenai. Children must not smoke.

5. Riverside warning この川で泳いではいけません。 Kono kawa de oyoide wa ikemasen. You must not swim in this river.

Notice the pattern: the plain form (いけない) appears in relaxed or personal statements; the polite form (いけません) suits public notices and formal rules.

Cultural note: Public spaces across Japan — parks, temples, museums — rely heavily on てはいけません for posted rules. A classic example is 芝生に入ってはいけません (shibafu ni haitte wa ikemasen, "Do not walk on the grass"), found in parks nationwide. Being able to read prohibition notices is one of the fastest practical payoffs of learning this grammar pattern.

Once you know how to state a rule with てはいけない, the next natural step is explaining the reason behind it — which is exactly what polite ので clauses let you do.

Ready to practise forming prohibition sentences with real-time feedback? Start a Free Trial lesson with Kind Japanese — one-on-one over LINE with a real teacher, whenever suits you.

Casual Speech: ちゃいけない and ちゃだめ

In natural spoken Japanese, は contracts into the preceding syllable and longer forms compress. Two shortened versions are extremely common in everyday conversation:

Contraction rule: - て + は → ちゃ (for verbs whose て-form ends in て) - で + は → じゃ (for verbs whose て-form ends in で)

For an even more informal register, replace いけない with だめ (dame, "no good"): - 食べちゃだめ (tabecha dame) — "don't eat that" - 飲んじゃだめ (nonja dame) — "don't drink that" - しちゃだめ (shicha dame) — "don't do that"

だめ softens the prohibition slightly and is especially common in parental speech and casual warnings between close friends. Use てはいけない in writing or formal speech; keep ちゃいけない and ちゃだめ for informal conversation.

Comparing Similar Prohibition Patterns

Japanese has a finely graded set of prohibition forms, and choosing the right one determines whether you sound natural or unnatural:

The key distinction: てはならない carries a heavier, more absolute weight — closer to "one must never…" than "you shouldn't." You will find it in laws, rulebooks, and literary works. In everyday Japanese, てはいけない is always the natural, neutral choice.

The opposite of てはいけない is てもいい (te mo ii), meaning "it is okay to" or "you may." Learning the prohibition and permission forms as a pair is the most efficient way to handle both directions of a rule in Japanese.

Mapping related patterns by formality, register, and nuance is a transferable skill — you will use the same analytical thinking when you reach Japanese comparative expressions, where register distinctions matter just as much.

Common Mistakes

1. Using the dictionary form instead of the て-form. Learners who are still consolidating te-form fluency sometimes write 食べるはいけない instead of 食べてはいけない. The て-form is non-negotiable — the entire construction fails without it. Drilling the conjugation table above until it is automatic eliminates this error.

2. Confusing てはいけない with てもいい. These are exact opposites. てはいけない means "must not do"; てもいい means "it is okay to do." The contrast is created by the particles: は + いけない (unacceptable) versus も + いい (fine). Mixing them up reverses the meaning entirely — a critical error in real communication.

3. Using てはならない in casual conversation. Learners who have absorbed a lot of textbook or literary Japanese sometimes default to てはならない. This sounds stiff and unnatural in everyday speech. Reserve it for formal written contexts.

4. Dropping the particle は. The standard construction requires は: 食べてはいけない, not 食べていけない. While the contracted form can drop は into ちゃ/じゃ in casual speech, simply omitting it in the full form is not standard.

5. Inconsistent politeness level. Mixing a plain てはいけない with polite speech elsewhere in the same utterance sounds awkward. Choose plain or polite at the start of a sentence and stay consistent throughout.

Practice Quiz

① Conjugate to てはいけない

② What does each sentence mean in English?

③ Translate into Japanese

Answers

FAQ

What does てはいけない mean in Japanese?

てはいけない (te wa ikenai) means "must not" or "not allowed to." It expresses prohibition by adding はいけない to any verb's て-form. The word いけない derives from 行けない ("cannot go / unacceptable"), giving the phrase the sense of "this action is not okay." It is a JLPT N5 grammar point.

What is the difference between てはいけない and てはならない?

てはいけない is the neutral, everyday form for prohibition — natural in both speech and writing. てはならない is formal and absolute, found in legal texts, official documents, and literary Japanese. In daily conversation always use てはいけない; てはならない in casual speech sounds stiff and unnatural.

How do you say "must not" casually in Japanese?

Contract て + は into ちゃ, or で + は into じゃ: 食べてはいけない becomes 食べちゃいけない (tabecha ikenai). For an even more relaxed tone, replace いけない with だめ to get 食べちゃだめ (tabecha dame). Both contracted forms are natural and common in spoken Japanese among friends and family.

What JLPT level is てはいけない?

てはいけない is JLPT N5 — the beginner foundation level. Mastering it alongside its opposite てもいい ("it is okay to") and its casual variants ちゃいけない and ちゃだめ gives you a complete, practical toolkit for expressing permission and prohibition at an early stage of learning.

Continue Learning

← Polite requests with てください → Explaining reasons politely with ので

This article is Lesson 57 of the Kind Japanese 100-day beginner curriculum.