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What is the main structural difference between مَفْعَلَانِ and مَفَاعِلُ?

Derived Nouns: Noun of Place and Time (Ism Dharf)

• The first is dual; the second is a broken plural
• Both are dual
• Both are plural
• The first is Ism Fa'il and the second is Ism Maf'ool