Recall the ending groups taught before. Prohibitive لَا applies the same jussive changes.
● A regular final consonant receives sukun: تَكْتُبُ ← لَا تَكْتُبْ.
● A form ending in inflectional nūn loses that nūn: تَقْتُلُونَ ← لَا تَقْتُلُوا.
● The feminine-plural pronominal nūn remains: تَكْتُبْنَ ← لَا تَكْتُبْنَ.
Do not delete every nūn: inflectional and pronominal nūn are different.