Aḥmadu is ordinarily diptote and has no tanwīn, while Zaydun is triptote. Both are proper names, so definiteness cannot explain the contrast.
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Contrast with Diptotes
Tanwīn: Five Types, Their Function, and Omission
Aḥmadu is ordinarily diptote and has no tanwīn, while Zaydun is triptote. Both are proper names, so definiteness cannot explain the contrast.