هَذَا خَاتَمٌ حَدِيدًا: khātamun is the predicate; ḥadīdan is accusative tamyīz.
هَذَا خَاتَمُ حَدِيدٍ: khātamu is both predicate and muḍāf; ḥadīdin is genitive muḍāf ilayh.
Meanings are similar but syntax is not. The book says the genitive construction is more frequent.
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At-Tamyīz: Clarifying Quantities and Sentence Relations
هَذَا خَاتَمٌ حَدِيدًا: khātamun is the predicate; ḥadīdan is accusative tamyīz.
هَذَا خَاتَمُ حَدِيدٍ: khātamu is both predicate and muḍāf; ḥadīdin is genitive muḍāf ilayh.
Meanings are similar but syntax is not. The book says the genitive construction is more frequent.