جَاءَنِي زَيْدٌ — subject, nominative: ḍamma and tanwīn.
رَأَيْتُ زَيْدًا — object, accusative: fatḥa and tanwīn.
مَرَرْتُ بِزَيْدٍ — governed by a preposition, genitive: kasra and tanwīn.
A reliable parsing sequence is: identify the noun's syntactic function, then identify the case marker. Naming a visible vowel alone does not complete the analysis.