زَيْدٌ is a proper name, yet it is fully declinable:
جَاءَ زَيْدٌ — Zayd came.
رَأَيْتُ زَيْدًا — I saw Zayd.
مَرَرْتُ بِزَيْدٍ — I passed by Zayd.
The three cases have their regular endings and tanwīn. A proper name does not automatically become a diptote merely because it is a name.
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Definiteness and Proper-Name Status in Diptote Rules
زَيْدٌ is a proper name, yet it is fully declinable:
جَاءَ زَيْدٌ — Zayd came.
رَأَيْتُ زَيْدًا — I saw Zayd.
مَرَرْتُ بِزَيْدٍ — I passed by Zayd.
The three cases have their regular endings and tanwīn. A proper name does not automatically become a diptote merely because it is a name.