زَيْدٌ قَائِمٌ — nominative mubtadaʾ and khabar.
إِنَّ زَيْدًا قَائِمٌ — accusative ism inna; nominative khabar.
كَانَ زَيْدٌ قَائِمًا — nominative ism kāna; accusative khabar.
Fāʿil, direct object and ism kāna are not interchangeable labels. A nominative noun after kāna is not simply classified as the verb’s ordinary doer.