«زَيْدٌ قَائِمٌ» has two nominative elements: mubtadaʾ and khabar.
«مَا زَيْدٌ قَائِمًا» has a nominative ism mā but an accusative khabar mā.
The predicate changes to fatḥah because this mā performs the governing action associated with laysa.
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Before and After Mā
The Predicate of Mā and Lā Resembling Laysa
«زَيْدٌ قَائِمٌ» has two nominative elements: mubtadaʾ and khabar.
«مَا زَيْدٌ قَائِمًا» has a nominative ism mā but an accusative khabar mā.
The predicate changes to fatḥah because this mā performs the governing action associated with laysa.