Grammar › Obligation › 〜てはいけない
〜てはいけない
"You must not / may not do that." Take the te-form, add はいけない — and hear how it shrinks to ちゃ／じゃ when people actually talk.
The short version
Take a verb's te-form, add は, then いけない / ならない / だめ: that's "must not". Polite is てはいけません. All three endings mean roughly the same "it won't do" — いけない is the everyday choice.
見る → 見てはいけない — must not look · casual
見てはいけません ／ 見てはだめ — polite / blunt
〜てはいけません — the polite ban
This is the form for rules, signs and instructions: "you may not…".
スマートフォンを使ってはいけません。
You must not use a smartphone. (使う → 使って + はいけません)
外に出てはいけません。
You must not go outside. (出る → 出て + はいけません)
〜てはいけない — plain form
Drop to plain いけない among friends or when narrating.
How it's really said — ちゃ / じゃ
🗣 ては → ちゃ, では → じゃ. In casual speech the はい clump gets crushed: 見てはだめ → 見ちゃだめ ("don't look"); 飲んではだめ → 飲んじゃだめ ("don't drink"). The verbs that make で in the te-form (〜んで, 〜いで) turn into じゃ; the rest into ちゃ. You still keep だめ／いけない on the end.
食べては だめ → 食べちゃだめ — don't eat it
飲んでは いけない → 飲んじゃいけない — must not drink
Watch out
てはいけない = don't; てもいい = go ahead. Same te-form, opposite meaning: 〜てはいけない forbids, while 〜てもいい permits. Swap は for も and a ban becomes a green light.
Next: 〜なくてはいけない — must → · 〜なくてもいい → · All grammar
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