Uktub is an imperative. Hallā taktubu is an exhortative particle plus an indicative imperfect. Similar intentions, different grammatical structures.
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Exhortation vs Imperative
Exhortative Particles: Urging, Reproach and Two Uses of Lawlā
Uktub is an imperative. Hallā taktubu is an exhortative particle plus an indicative imperfect. Similar intentions, different grammatical structures.