An indefinite nominative noun followed by agreeing indefinite nominative mubtasimun gives an adjective. With definite aṭ-ṭālibu, mubtasiman is an accusative ḥāl describing the manner of arrival.
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Case 5: Adjective versus Circumstantial Accusative
Advanced Parsing Workshop: Integrated Grammar
An indefinite nominative noun followed by agreeing indefinite nominative mubtasimun gives an adjective. With definite aṭ-ṭālibu, mubtasiman is an accusative ḥāl describing the manner of arrival.