〜てはいけない expresses prohibition. It means “you must not …” or “it is not allowed to …”. It is made with the verb’s て-form + は + いけない. Key points: • Verb て-form + はいけない • Strong prohibition or rule • Polite form: 〜てはいけません / 〜てはいけないです • Often used for rules and warnings For example, “ここでタバコを吸ってはいけません” means “You must not smoke here.” This pattern is strong, so use it for rules or strict advice. For softer language, people might say だめです, but てはいけない is clear and direct. Remember: て-form + は + いけない. It is the opposite of 〜てもいい (permission). As a beginner, use it in set phrases and rules you often see: “入ってはいけません” (Do not enter), “食べてはいけません” (You must not eat).