جَاءَ زَيْدٌ — Zayd came.
جَاءَ: past-tense verb, built on fatḥa.
زَيْدٌ: subject (fāʿil), nominative, marked by an overt ḍamma with tanwīn because the proper noun is fully declinable.
The function determines the case; the noun’s declension class determines the ending.