In هَلْ هَذَا كِتَابٌ؟ hal is interrogative; hādhā is a demonstrative subject in the nominative position; kitābun is the nominative predicate.
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Full Parsing of a Nominal Question
The Two Interrogative Particles: Hamza and Hal
In هَلْ هَذَا كِتَابٌ؟ hal is interrogative; hādhā is a demonstrative subject in the nominative position; kitābun is the nominative predicate.