زَيْدٌ مَحْبُوبٌ.
Meaning: Zayd is loved.
Zayd is the subject of the nominal sentence; maḥbūbun is the nominative predicate. A concealed pronoun in the passive participle functions as its passive subject and refers to Zayd.
The active participle’s implicit pronoun identifies the doer; the passive participle’s implicit pronoun identifies the undergoer.