To derive a direct imperative from يَسْتَعِينُ, the imperfect prefix is removed and the form becomes jussive. With this hollow stem, the long medial vowel contracts before the resulting consonantal closure: اِسْتَعِنْ.
يَسْتَعِينُ → اِسْتَعِنْ
تَسْتَعِينُ → لَا تَسْتَعِنْ
By contrast, يَسْتَعِينُ is the ordinary indicative imperfect. Copying it unchanged as a command or prohibition would ignore the relevant inflection and hollow-verb alteration.
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Why the Imperative Is Istaʿin
Derived Hollow Wāw and Yāʾ Verbs: Short Paradigms and Morphological Reasoning
To derive a direct imperative from يَسْتَعِينُ, the imperfect prefix is removed and the form becomes jussive. With this hollow stem, the long medial vowel contracts before the resulting consonantal closure: اِسْتَعِنْ.
يَسْتَعِينُ → اِسْتَعِنْ
تَسْتَعِينُ → لَا تَسْتَعِنْ
By contrast, يَسْتَعِينُ is the ordinary indicative imperfect. Copying it unchanged as a command or prohibition would ignore the relevant inflection and hollow-verb alteration.