هَذَا زَيْدٌ قَائِمًا
Translation: This is Zayd, in a standing state.
هَذَا: demonstrative, indeclinable, in the nominative position as topic.
زَيْدٌ: nominative predicate.
قَائِمًا: accusative ḥāl describing the person pointed to; the act of pointing is a semantic governor.
A nominative qāʾimun would open a different analysis. The author deliberately gives the accusative qāʾiman here: it is ḥāl.