The Three Forms of the Subject

Rules of the Fāʿil: Explicit Nouns, Pronouns, Number and Gender

Explicit noun: Zayd in «ذَهَبَ زَيْدٌ».
Visible pronoun: the tāʾ in «ضَرَبْتُ زَيْدًا».
Concealed pronoun: the understood «هُوَ» in «زَيْدٌ ذَهَبَ».

All three are subjects and occupy the nominative role.