«اِشْتَرَيْتُ كِتَابًا»: kitāban is the direct object of “bought.”
«عِنْدِي عِشْرُونَ كِتَابًا»: kitāban is number tamyīz clarifying what was counted.
Both nouns are accusative, but they perform different grammatical functions.
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Tamyīz vs Direct Object
At-Tamyīz: Clarifying Quantities and Sentence Relations
«اِشْتَرَيْتُ كِتَابًا»: kitāban is the direct object of “bought.”
«عِنْدِي عِشْرُونَ كِتَابًا»: kitāban is number tamyīz clarifying what was counted.
Both nouns are accusative, but they perform different grammatical functions.