GrammarJLPT N5
→ must not
Must Not / May Not (Prohibition)
JLPT N5Fixed phraseNeutral
～てはいけない is used to express prohibition, meaning "must not" or "may not." It tells the listener that a certain action is not allowed or is forbidden. The grammar point is formed by attaching てはいけない to the て-form of a verb. It is commonly used by parents, teachers, or anyone in a position of authority to set rules, but it can also express general social rules or personal moral judgments. In casual spoken Japanese, ては often contracts to ちゃ (or じゃ after で-form verbs), producing forms like 食べちゃいけない or 飲んじゃいけない. A similar expression, ～てはだめ, is more casual and commonly used among friends or family, while ～てはならない is more formal and literary, often found in written rules or laws. Compared to ～ないでください, which is a polite request not to do something, ～てはいけない carries a stronger sense of prohibition and authority.
Examples
Example #1
教室で食べ物を食べてはいけない。
Here, 食べてはいけない is used to state a rule that eating in the classroom is prohibited. The speaker (likely a teacher) is establishing a clear prohibition. The て-form of 食べる is 食べて, and はいけない is attached to express that the action is not permitted.You must not eat food in the classroom.
Structures
- Verb
- Verb (て-form) + はいけない
Common mistakes
A common mistake is confusing ～てはいけない (prohibition: must not do) with ～なくてはいけない (obligation: must do). These are essentially opposites, so mixing them up leads to the opposite meaning. Another frequent error is forgetting to use the て-form of the verb before はいけない, such as incorrectly saying 食べるはいけない instead of 食べてはいけない. Learners also sometimes use ～てはいけない when they simply want to make a polite request, in which case ～ないでください would be more appropriate and less authoritative. Finally, in casual speech, learners may not realize that ては contracts to ちゃ (and では to じゃ), so they may sound overly stiff in informal conversations by always using the full form.
Related
～てはならない～てはだめ～なくてはいけない～ないでください～てもいい～べきではない