This pattern is used to state that something is absolutely not allowed or should never be done. It carries a strong, often moral or societal weight, similar to saying "must not" or "cannot be allowed" in English. It is more formal and emphatic than てはいけない, and is often found in rules, laws, or written guidelines, though it can be used in speech to express strong personal conviction.

You will frequently encounter this pattern in set phrases like 言ってはならない (must not say), 忘れてはならない (must not forget), or 見てはならない (must not look). The は adds a contrastive nuance, emphasizing that the action is particularly forbidden. In everyday conversation, it might sound a bit stiff; people often use てはいけない or ちゃだめ instead.

Note that はならない literally means "does not become," so the pattern implies that if you do that action, things will not be right — it is not merely a rule, but a fundamental principle.