In inna Zaydan, Zayd is accusative as ism inna. In innamā Zaydun, mā blocks inna from governing and Zayd is nominative. That blocking effect distinguishes mā kāffa from simply additional mā.
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Blocking Mā Is Not Simply Extra
Additional Particles: Seven Forms, Emphasis and Syntactic Case
In inna Zaydan, Zayd is accusative as ism inna. In innamā Zaydun, mā blocks inna from governing and Zayd is nominative. That blocking effect distinguishes mā kāffa from simply additional mā.