なくてはならない

なくてはならない expresses a strong obligation or necessity: you must do something, often because of rules, responsibilities, or personal commitment. It sounds formal or written, and is commonly used in writing, announcements, or polite conversation. It emphasizes that not doing it would be unacceptable or have negative consequences. Note that the verb before なくて is in the て-form negative (undoing the action) + はならない, but in practice you translate as 'must' or 'have to'. Typical contexts include duties, deadlines, promises, or requirements one cannot ignore.