Lā al-nāhiyah forbids an action and makes the imperfect jussive. Ordinary negative lā (lā al-nāfiyah) negates a statement; by itself it does not impose the jussive mood.
لَا تَكْتُبْ — do not write!; the imperfect ends in sukūn.
لَا تَكْتُبُ — you do not write; the imperfect remains indicative.
Unvowelled orthography may look identical. Read the mood, intended force and context together rather than treating every occurrence of lā as a command not to act.
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Prohibitive lā versus ordinary negative lā
The Prohibitive: Active and Passive Paradigms with Emphatic Nūn
Lā al-nāhiyah forbids an action and makes the imperfect jussive. Ordinary negative lā (lā al-nāfiyah) negates a statement; by itself it does not impose the jussive mood.
لَا تَكْتُبْ — do not write!; the imperfect ends in sukūn.
لَا تَكْتُبُ — you do not write; the imperfect remains indicative.
Unvowelled orthography may look identical. Read the mood, intended force and context together rather than treating every occurrence of lā as a command not to act.