Ḍarb is a maṣdar by word type.
In ضَرَبْتُ ضَرْبًا the verbal noun functions as an absolute object and is accusative.
Not every maṣdar is always an absolute object: in الضَّرْبُ مُؤْلِمٌ it is a nominative mubtadaʾ.
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Maṣdar versus Its Grammatical Role
The Accusatives: Al-Mafʿūl al-Muṭlaq
Ḍarb is a maṣdar by word type.
In ضَرَبْتُ ضَرْبًا the verbal noun functions as an absolute object and is accusative.
Not every maṣdar is always an absolute object: in الضَّرْبُ مُؤْلِمٌ it is a nominative mubtadaʾ.