When a whole clause is khabar, the entire clause occupies the nominative syntactic position. That does not make each word nominative.
In «زَيْدٌ قَامَ أَبُوهُ», the past verb is indeclinable, the subject is nominative, and the entire verbal clause serves as the khabar in the nominative syntactic slot.
Opening interactive controls...
Types of Khabar and the Linking Pronoun
When a whole clause is khabar, the entire clause occupies the nominative syntactic position. That does not make each word nominative.
In «زَيْدٌ قَامَ أَبُوهُ», the past verb is indeclinable, the subject is nominative, and the entire verbal clause serves as the khabar in the nominative syntactic slot.