كَتَبْتُ — -tu: subject, nominative position.
رَأَيْتُهُ — -hu: object, accusative position.
مَرَرْتُ بِهِ — -hi: genitive after preposition.
كِتَابُهُ — -hu: genitive as annexed complement.
إِنَّهُ صَادِقٌ — -hu: noun of inna, accusative position.
زَيْدٌ هُوَ الْقَائِمُ — huwa: separator, no independent case position on the common analysis.
Always coordinate governor, position, and meaning.