てはならない

てはならない means "must not". It is the formal, written way to forbid something: laws, rules, speeches, moral statements. It goes on the て-form of the verb, followed by は.

A な-adjective or a noun takes であっては.

てはならない or てはいけない

Both mean "must not". てはいけない is what you say: a parent to a child, a sign in a classroom. てはならない states a principle that holds for everyone, and in conversation it sounds solemn or stiff.

The polite form てはなりません turns up in formal speech and in rules written for the reader.

てはならない or なければならない

なければならない means the opposite: "must do". The difference is whether the verb in front is negative.