〜てはならない
must not (formal)
Verb て-form + はならない
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てはならない is the formal, written version of てはいけない: この事件を忘れてはならない, "we must never forget this incident".
It's used for rules, principles and moral statements rather than personal instructions: in laws, speeches, notices, essays and the news. It often states what society or people in general must not do: 人を見た目で判断してはならない, "you must not judge people by appearances".
With 決して, it becomes very strong: 決してあきらめてはならない, "you must never give up".
Watch the similar pair: なくてはならない means "must", and てならない (without は) means "can't help feeling".
Examples
この事件を忘れてはならない。
We must never forget this incident.
試験中は、話してはならない。
Talking is not permitted during the exam.
同じ過ちを繰り返してはならない。
We must not repeat the same mistake.
人を見た目で判断してはならない。
You must not judge people by their appearance.
決してあきらめてはならない。
You must never give up.