Overview
Both of these patterns tell someone that an action is forbidden — “you must not,” “you may not,” “don’t you dare.” They are not two different rules with two different meanings; they are the same rule wearing different clothes. てはいけない is the full, standard form you meet in textbooks, signs, and polite speech. ちゃいけない・じゃいけない is that exact same form after casual speech has squashed it: ては collapses into ちゃ, and では collapses into じゃ. The shorthand for students: てはいけない = the neat, complete, “public” form; ちゃいけない・じゃいけない = the relaxed, contracted, “friends-and-family” form. The two are constantly confused not because their meanings differ — they don’t — but because beginners often can’t see that 食べちゃいけない is simply 食べてはいけない with the middle worn smooth, and they panic that ちゃ and じゃ are brand-new grammar. They aren’t. The boundary below is about sound and register, not meaning, and getting that straight removes half the fear. The entries lead with a simple example, then a funny one.
てはいけない — “must not / may not (standard prohibition)”
Explanation
てはいけない attaches to the て-form of a verb and states that doing that action is not allowed. 授業中に寝てはいけない means “you must not sleep during class” — the sleeping is forbidden, flatly and clearly. The nuance is a rule coming down from somewhere: a law, a teacher, a parent, a sign, or plain common sense. It is the natural form for stating prohibitions that everyone is expected to respect — 芝生に入ってはいけない (“you must not walk on the grass”), 嘘をついてはいけない (“you must not tell lies”). Because it is complete and unslurred, it carries a slightly firm, official tone, which is exactly why signs and announcements love it.
Grammatically the build is fixed: て-form of the verb + は + いけない. The verb must first become its て-form (食べる → 食べて, 飲む → 飲んで, する → して, 来る → 来て), and only then do you bolt on はいけない. The は here is the topic particle は, pronounced wa, not “ha” — this trips up learners who read it letter by letter. The いけない half is the negative potential of いく in a frozen idiom (“it won’t do / it won’t go well”), so the literal image is “as for doing X, it won’t do.” You can swap the ending without changing the meaning much: てはいけない (neutral), てはならない (stiffer, written, legal/moral), and てはだめ (softer, more personal). To be polite, いけない becomes いけません: 入ってはいけません.
The register is neutral-to-firm and works in both writing and speech. The teaching anchor is the full, uncontracted form: てはいけない is what you write, what you read on signs, and what you say when you are being clear or a little formal. It differs from ちゃいけない・じゃいけない only in that the latter has been contracted for casual speech. If the student is writing, reading a notice, or speaking carefully, てはいけない is the fit.
Examples
授業中にスマホを見てはいけません。 — You must not look at your smartphone during class.
🍜 「ダイエット中だから、ラーメンを食べてはいけません。」でも、ラーメンはとてもおいしいです。だから、今日はダイエットをしません。明日からがんばります。 — “I’m on a diet, so I must not eat ramen.” But ramen is very delicious. So today I won’t diet. I’ll do my best starting tomorrow.
Common mistake
Don’t forget to convert the verb to its て-form first — the pattern attaches to て, never to the dictionary form. ✗ 食べるてはいけない is wrong; it must be 食べてはいけない. And remember the は is the particle wa: writing or saying it as “ha” marks you instantly as a beginner. If you want the same meaning in casual speech, contract it — that’s where ちゃいけない・じゃいけない comes in.
ちゃいけない・じゃいけない — “must not / mustn’t (casual, spoken)”
Explanation
ちゃいけない・じゃいけない means precisely what てはいけない means — “you must not do this” — but it is the contracted, conversational version you actually hear between friends, from parents to kids, and in casual daily life. ここで遊んじゃいけない (“you can’t play here”) and 遅刻しちゃいけない (“you mustn’t be late”) are everyday spoken prohibitions. Nothing about the degree of prohibition changes; only the formality drops. This is the form of nagging older brothers, exasperated mothers, and friends warning each other — warm, immediate, and unbuttoned.
Grammatically, the whole thing is a sound contraction of てはいけない, and there is one clean rule that decides ちゃ versus じゃ. It depends on whether the verb’s て-form ends in て or で:
- Verbs whose て-form ends in て → ては becomes ちゃ: 食べては → 食べちゃ, 見ては → 見ちゃ, しては → しちゃ.
- Verbs whose て-form ends in で → では becomes じゃ: 飲んでは → 飲んじゃ, 読んでは → 読んじゃ, 遊んでは → 遊んじゃ.
So the voiced で-group takes the voiced じゃ, and the plain て-group takes ちゃ — the pairing follows the sound, not the meaning. After that contraction you simply add いけない (or the even more casual だめ): 食べちゃいけない = 食べちゃだめ, 飲んじゃいけない = 飲んじゃだめ. Because it’s casual, the polite いけません is possible but less common; you’ll more often hear ちゃだめ / じゃだめ or just a sharp ちゃだめ！
The register is casual and spoken — friends, family, children, informal chat. The teaching anchor is contraction for speech: ちゃ = squashed ては, じゃ = squashed では. It differs from てはいけない only in formality. If the student is speaking casually or writing dialogue, this is the natural fit; if they need the full or written form, they should un-squash it back to てはいけない.
Examples
そんなにお菓子を食べちゃだめだよ。 — You mustn’t eat that many sweets, you know.
🐱 病院に「猫にさわっちゃいけません」と書いてありました。でも、猫はとてもかわいいです。猫も私を見ました。だから、少しだけさわりました。 — The sign at the clinic said “you must not touch the cat.” But the cat is very cute. The cat looked at me too. So I touched it just a little.
🎮 テストは明日です。母が「ゲームで遊んじゃだめ」と言いました。でも、これはゲームじゃないです。日本語の勉強です。私はゲームで日本語を勉強しています。 — The test is tomorrow. My mother said “don’t play video games.” But this isn’t a game. It’s Japanese study. I’m studying Japanese with a game.
Common mistake
Don’t mix up which contraction a verb takes — the choice of ちゃ vs じゃ is dictated by the て-form, not by your preference. ✗ 飲んちゃいけない is wrong because 飲む becomes 飲んで (で-ending), so it must be 飲んじゃいけない. Likewise ✗ 食べじゃいけない is wrong; 食べる becomes 食べて (て-ending), so it’s 食べちゃいけない. If you can form the て-form correctly, the contraction falls out automatically: て→ちゃ, で→じゃ.
How to choose
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