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

Although なくてはならない 'must do' is introduced to many learners early on, てはならない by itself carries the meaning of 'must not do', or 'cannot do'. This structure is a combination of the conjunction particle て, は, and the negated form of the う-Verb なる. Despite this ならない by itself is often considered a standalone word which may indicate that something is 'not possible', 'not permitted', or 'cannot be helped', depending on what type of word it is following.

When following verbs and ては, ならない can only mean 'cannot', or 'must not'.

Caution

As a reminder, in the order from most casual to most formal, てはだめ, てはいけない, and てはならない will often be seen in sentences with no significant difference in meaning.

Due to the ならない variant being the most formal, it is also the most likely to be seen in writing or polite speech.