«مَا إِنْ زَيْدٌ قَائِمٌ» uses added in with sukūn; Zayd remains a mubtadaʾ.
«إِنَّ زَيْدًا قَائِمٌ» uses inna with shaddah, a particle that makes Zayd its accusative noun.
Always distinguish these two particles by form, meaning and governance.
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In versus Inna
The Predicate of Mā and Lā Resembling Laysa
«مَا إِنْ زَيْدٌ قَائِمٌ» uses added in with sukūn; Zayd remains a mubtadaʾ.
«إِنَّ زَيْدًا قَائِمٌ» uses inna with shaddah, a particle that makes Zayd its accusative noun.
Always distinguish these two particles by form, meaning and governance.