Urīdu allā ataʾakhkhara has infinitival an + negative lā, so the imperfect is subjunctive. In exhortative allā tataʾakhkharu?, it is indicative.
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Another Construction Spelled Allā
Exhortative Particles: Urging, Reproach and Two Uses of Lawlā
Urīdu allā ataʾakhkhara has infinitival an + negative lā, so the imperfect is subjunctive. In exhortative allā tataʾakhkharu?, it is indicative.