ضَرَبْتُ زَيْدًا: Zayd is a direct object receiving the action.
سِرْتُ وَالنَّهْرَ: the river is an accompaniment complement, not the thing walked upon by the verb.
Both may be accusative, but their semantic roles differ. These are comparative teaching examples.
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Al-Mafʿūl Maʿahu: Accompaniment and Coordination
ضَرَبْتُ زَيْدًا: Zayd is a direct object receiving the action.
سِرْتُ وَالنَّهْرَ: the river is an accompaniment complement, not the thing walked upon by the verb.
Both may be accusative, but their semantic roles differ. These are comparative teaching examples.