After innamā in a verbal clause, al-muʿallimu is the nominative subject of ḥaḍara, not the accusative subject of inna.
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Subject after Innamā
Inna and Its Sisters: Six Particles, Meanings, Government and Exceptions
After innamā in a verbal clause, al-muʿallimu is the nominative subject of ḥaḍara, not the accusative subject of inna.