What two changes distinguish لَمْ يَكْتُبُوا from لَا يَكْتُبُونَ?

Negating the Imperfect with Lan and Lam: Subjunctive, Jussive and Full Paradigms

• It becomes passive and singular
• It becomes past-negative, and inflectional nūn drops
• It becomes future-negative, and an extra nūn appears
• It shifts from third-person plural to first-person plural