مَا زَيْدٌ إِلَّا قَائِمٌ
Translation: Zayd is only standing.
مَا: negative particle, non-working in this exceptive construction.
زَيْدٌ: nominative mubtadaʾ.
إِلَّا: particle marking exclusivity.
قَائِمٌ: nominative khabar.
This is a supplemental teaching rule, not a quoted line from the book.