〜なくてはいけない expresses obligation. It means “must” or “have to.” It is the opposite of 〜てはいけない. Key points: • Verb ない-form + くてはいけない • Means you must do something • Polite form: 〜なくてはいけません • Often used for rules and responsibilities For example, “勉強しなくてはいけません” means “I must study.” “早く起きなくてはいけません” means “I have to wake up early.” It sounds strong, so use it when the obligation is real. In casual speech, people often shorten it to 〜なきゃ, but at N5 you should use the full form. To make this pattern, first change the verb to the ない-form, then add くてはいけない. For する, it becomes しなくてはいけない. Practice with common verbs to get used to the form and meaning.