لَا تَكْتُبُ — لَا تَكْتُبْ
Lā taktubu = you do not write, with non-jussive negative lā (indicative). Lā taktub = do not write, with prohibitive lā (jussive). This keeps the same subject for a fair comparison.
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Two Functions of Lā
The Indicative Imperfect and Its Governor
لَا تَكْتُبُ — لَا تَكْتُبْ
Lā taktubu = you do not write, with non-jussive negative lā (indicative). Lā taktub = do not write, with prohibitive lā (jussive). This keeps the same subject for a fair comparison.