てはならない

てはならない expresses a strong prohibition or rule, often found in formal instruction, warnings, or official guidelines. It marks actions that are strictly forbidden and carry a sense of moral or normative obligation, stronger than a simple negative instruction. It is common in rules, safety signs, and formal communication, and it can feel stern or authoritative. It should not be used for casual refusals or everyday warnings (more common are 〜ちゃいけない or 〜てはいけない in informal to neutral contexts). Examples are typically about behavior in a place, safety, honesty, or compliance with rules.