كَانَ زَيْدٌ قَائِمًا
Translation: Zayd was standing.
كَانَ: past-tense incomplete verb, built on fatḥa.
زَيْدٌ: nominative ism kāna, visible ḍamma.
قَائِمًا: accusative khabar kāna, visible fatḥa.
Zayd retains the nominative ending, but the predicate changes from «قَائِمٌ» to «قَائِمًا». Focus on this actual case change.