modality N3 common politewritten
〜ではならない — must not ~ (formal prohibition
〜ではならない ・ ではならない
Builds on て形
A full written guide for 〜ではならない is on the way.
This point is mapped in the grammar tree, but its examples, formation table and usage notes haven't been written yet. The fastest way to learn it now is to meet it in real sentences inside Jengo.
See 〜ではならない in real sentences
Jengo shows 〜ではならない the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.
Study it in JengoSources Compiled with reference to A Dictionary of Basic Japanese Grammar. Editorial confidence: medium.