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What does the grammatical term "Thulaathi Mazeed Feeh" (ثلاثی مزید فیہ) mean?

Introduction to Derived Trilateral Verbs (Thulaathi Mazeed Feeh)

• Verbs with three root letters and zero extra letters
• Verbs with three root letters and one or more extra letters
• Verbs with exactly four root letters
• Non-conjugatable verbs