The second cause in the taught pair is a verb-like pattern. The adjective أَحْمَرُ is on the pattern أَفْعَلُ, a pattern also associated with verbs. Likewise أَكْبَرُ follows أَفْعَلُ.
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Cause 2 in This Pair: The Pattern أَفْعَلُ
Causes of Ghayr Munsarif: Wasf and the Pattern Af'al
The second cause in the taught pair is a verb-like pattern. The adjective أَحْمَرُ is on the pattern أَفْعَلُ, a pattern also associated with verbs. Likewise أَكْبَرُ follows أَفْعَلُ.