Zaydun nafsuhu: nafsuhu is emphasis; Zayd himself came.
Zaydun akhūka: akhūka is badal; the intended Zayd is your brother.
Both are nominative but perform different discourse functions. This is an additional comparison.
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Badal or Emphasis?
Badal: Definition, Four Types, and Parsing
Zaydun nafsuhu: nafsuhu is emphasis; Zayd himself came.
Zaydun akhūka: akhūka is badal; the intended Zayd is your brother.
Both are nominative but perform different discourse functions. This is an additional comparison.