てはいけない

1. Formation & Logic:
To create this pattern, take the て form of a verb, adjective, or noun and add てはいけない (plain) or てはいけません (polite). Note that the は in this pattern is the topic marker and must be pronounced as 「わ」, not 「は」.

Literally, this breaks down as: て-form (marking the action) + は (making that action the topic) + いけない (cannot go). So it literally means 'As for doing [action], it cannot go' (meaning it is unacceptable).

• 食べる（たべる）→ 食べてはいけない (must not eat)
• 飲む（のむ）→ 飲んではいけない (must not drink) - Note: For words whose て-form ends in で, the pattern naturally becomes ではいけない.
• 高い（たかい）→ 高くてはいけない (must not be expensive)
• 静か（しずか）→ 静かではいけない (must not be quiet) - Note: な-adjectives follow the exact same rule as nouns.
• 未成年（みせいねん）→ 未成年ではいけない (must not be a minor) - Note: For nouns and な-adjectives, the variant 〜であってはいけない (e.g., 未成年であってはいけない) is also very common and sounds slightly more natural or formal.

Orthography Note: Because いけない acts as an auxiliary verb in this pattern, it should always be written in hiragana. Writing it in kanji (〜ては行けない) is discouraged as it can be confused with the literal physical inability to go somewhere.

2. Nuance and Formality Spectrum:
When used with verbs, this is a direct and authoritative prohibition. However, when used with adjectives and nouns (states), it does not carry the nuance of an authoritative scolding. Instead, it expresses a necessary condition or requirement (e.g., 高くてはいけない = It must not be expensive). The level of formality changes depending on the ending:

• 〜てはだめ (だ/です): Casual/Colloquial. Often used with だ or です. Used with friends or children.
• 〜てはいけない: Standard/Neutral. Used by authority figures or on signs.
• 〜てはならない: Formal/Written. Used in laws, regulations, or formal speeches.

3. Past Tense & Regret:
The final words (いけない, だめ, ならない) conjugate like normal adjectives or verbs to express the past tense. For example, 食べてはいけなかった means 'was not allowed to eat' or 'was not supposed to eat'. Importantly, native speakers frequently use 〜てはいけなかった to express deep personal regret, self-reproach, or the realization of a mistake (e.g., 「言ってはいけなかった」 - I shouldn't have said that). In this sense of regret, it is often used interchangeably with 〜べきではなかった.

4. Social Context:
Avoid using this toward superiors or strangers unless you are explaining a strict rule, as it can sound confrontational. For softer requests, use 〜ないでください (Please don't...) instead. In casual speech, 〜ては is often contracted to 〜ちゃ (e.g., 食べちゃいけない) and 〜では is contracted to 〜じゃ (e.g., 飲んじゃいけない). Furthermore, these contractions are almost always combined with だめ (e.g., 食べちゃだめ, 飲んじゃだめ) when speaking with friends or children. In very casual conversation, だめ or いけない is often dropped entirely, leaving just the contraction (e.g., 「こら、食べちゃ！」 = 'Hey, you mustn't eat that!').