Under the concealed-subject pattern, the explanatory tamyīz is an indefinite accusative. Contrast the al-definite overt nominative subject pattern.
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Why the Tamyīz Is Indefinite and Accusative
Verbs of Praise and Blame: Niʿma, Ḥabbadhā, Biʾsa, Sāʾa
Under the concealed-subject pattern, the explanatory tamyīz is an indefinite accusative. Contrast the al-definite overt nominative subject pattern.