What it means
てはならない expresses a strict prohibition: an action must not be performed or a situation must not be allowed to occur. It is commonly used in written rules, regulations, formal instructions, and statements of principle. Compared with てはいけない, it sounds more formal and authoritative. The polite counterpart is てはなりません.
Usage note
てはならない is relatively formal and can sound stiff in ordinary conversation. Use てはいけない for a more common everyday prohibition.
How to use it
Verb (て-form)はなりません
この規則を無視してはなりません。
This rule must not be ignored.
Verb (て-form)はならない
この規則を無視してはならない。
This rule must not be ignored.