«مَا زَيْدٌ قَائِمًا»: noun first; mā governs, so khabar is accusative.
«مَا قَائِمٌ زَيْدٌ»: predicate first; mā does not govern, so both predicate and mubtadaʾ are nominative.
Case endings help identify the construction rather than just word order.
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Two Word Orders, Two Case Analyses
The Predicate of Mā and Lā Resembling Laysa
«مَا زَيْدٌ قَائِمًا»: noun first; mā governs, so khabar is accusative.
«مَا قَائِمٌ زَيْدٌ»: predicate first; mā does not govern, so both predicate and mubtadaʾ are nominative.
Case endings help identify the construction rather than just word order.