N2
てはならない
Must not, Cannot, Should not
お酒を飲んで運転してはならない。
You must not drink and drive.
Explanation
「てはならない」 means 'must never'. From the standpoint of social norms, law, morals or lessons of history, it makes a strong, universal prohibition: 「戦争の悲劇を風化させてはならない」 (We must never let the tragedy of war fade). The tone is solemn, used in rules, pledges, speeches, or serious warnings from elders. It is not for casual reminders to people around you, such as 'don't run in the hall'.
Mixed up with
「てはならない」: the heaviest ban; a matter of social, legal or moral principle.
「てはいけない」: the general ban, teacher to student, parent to child: 「ここで走ってはいけません」 (You must not run here).
「てはだめだ/ちゃだめ」: the close, direct 'no good' of everyday speech.