Ḍaraba: First Recognize the Verb

Diptote Causes — Verb Patterns and Their Conditions

ضَرَبَ زَيْدٌ. — Zayd struck.

ضَرَبَ: past-tense verb, built on fatḥa.
زَيْدٌ: subject (fāʿil), nominative with ḍamma.

Here ḍaraba is neither a person’s name nor a diptote noun; it is an actual verb.