The book illustrates the working construction مَا زَيْدٌ قَائِمًا. In other attested usage, mā can be non-working, with a nominative predicate as in مَا زَيْدٌ قَائِمٌ.
Do not label that other construction universally ungrammatical. Here we are testing the specific working mā presented by the book.
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Ism of Mā and Lā Resembling Laysa; Khabar of Lā of Generic Negation
The book illustrates the working construction مَا زَيْدٌ قَائِمًا. In other attested usage, mā can be non-working, with a nominative predicate as in مَا زَيْدٌ قَائِمٌ.
Do not label that other construction universally ungrammatical. Here we are testing the specific working mā presented by the book.