هَذَا كِتَابٌ.
Translation: This is a book.
هَذَا: subject; كِتَابٌ: nominative predicate.
هَذَا الْكِتَابُ مُفِيدٌ.
Translation: This book is useful.
هَذَا: subject; الْكِتَابُ: nominative apposition; مُفِيدٌ: predicate.
Definiteness and clause structure change the function of “kitāb”.