Hallā is a non-governing exhortative particle. Taktubu is indicative with implied subject anta; al-risālata is its accusative object.
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Parsing Hallā with an Imperfect
Exhortative Particles: Urging, Reproach and Two Uses of Lawlā
Hallā is a non-governing exhortative particle. Taktubu is indicative with implied subject anta; al-risālata is its accusative object.