In مَا زَيْدٌ قَائِمًا, Zaydun is not the khabar. It is what the statement is about and is the ism of mā.
Qāʾiman conveys the assertion and is its khabar. The same relationship applies to the source’s lā example.
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Frequent Mistake: Switching Ism and Khabar
Ism of Mā and Lā Resembling Laysa; Khabar of Lā of Generic Negation
In مَا زَيْدٌ قَائِمًا, Zaydun is not the khabar. It is what the statement is about and is the ism of mā.
Qāʾiman conveys the assertion and is its khabar. The same relationship applies to the source’s lā example.