What does “إِذَا نُكِّرَ صُرِفَ” teach in this passage?

Restoring Declension: Indefinite Names, Al- and Iḍāfah

• A name-dependent diptote can regain full declension when indefinite
• Every indefinite noun is diptote
• Every accusative noun takes tanwīn
• Al- cancels the nominative