Lawlā al-maṭaru lajaffa al-zarʿu means that rain prevented the crop from drying. It is counterfactual, not an exhortation.
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Example of Counterfactual Lawlā
Exhortative Particles: Urging, Reproach and Two Uses of Lawlā
Lawlā al-maṭaru lajaffa al-zarʿu means that rain prevented the crop from drying. It is counterfactual, not an exhortation.