Count the root letters, not every visible letter.
● A root with 3 original letters is ثلاثی.
● A root with 4 original letters is رباعی.
● If the first past-tense form contains only the 3 root letters, it is ثلاثی مجرد.
● If the first past-tense form contains one or more letters in addition to those 3 root letters, it is ثلاثی مزید فیہ.
According to this book, roughly 99% of verb roots are triliteral. Treat this as the book's approximation, not as a universal statistic.
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The Six Babs of Basic Triliteral Verbs
Count the root letters, not every visible letter.
● A root with 3 original letters is ثلاثی.
● A root with 4 original letters is رباعی.
● If the first past-tense form contains only the 3 root letters, it is ثلاثی مجرد.
● If the first past-tense form contains one or more letters in addition to those 3 root letters, it is ثلاثی مزید فیہ.
According to this book, roughly 99% of verb roots are triliteral. Treat this as the book's approximation, not as a universal statistic.