～ては成(な)らないMust not, Cannot, Should not

These express opposites: なければならない 'must' vs. てはならない 'must not'. For example, 行かなければならない 'must go' is the opposite of 行ってはならない 'must not go'. Both are formal and typically used in writing or structured speech. Casual speech favors shorter forms like なきゃ and ちゃ.

These are opposites: なければいけない expresses obligation (i.e. 'must'), while てはならない means prohibition (i.e. 'must not'). For example, 行かなければいけない ('You must go.') vs. 行ってはならない ('You must not go.'). Both are formal and common in writing or structured speech, while in conversation casual forms like なきゃ and ちゃ are more common.

Both てはいけない and てはならない express prohibition, saying something 'must not' or 'cannot' be done. The two forms are equivalent in meaning, with the only difference being that ならない is slightly more formal and therefore more often found in the written language, such as in rules or regulations.

なくてもいい means there is no obligation to do something, like the English 'you don't have to (A)' (or literally, 'it's okay even without'). For example, 食べなくてもいい means 'You don't have to eat it.' In contrast, てはならない expresses prohibition, like 'must not (A)' or 'cannot (A)'. For example, 食べてはならない means 'You must not eat it.'.