تَكْتُبُ يَا زَيْدُ.
Taktubu is an indicative present verb with concealed anta as subject; yā is the vocative particle; Zaydu is a singular proper vocative built on ḍamm in the accusative position.
زَيْدٌ يَكْتُبُ.
Zayd is the initial subject; yaktubu is the verb with concealed huwa; the verbal clause is the predicate.
فَاطِمَةُ تَكْتُبُ.
Taktubu contains a concealed hiya; the verbal clause predicates Fāṭimah.