Simple triliteral place/time nouns do not all share the same middle vowel. مَفْعَلٌ has fatḥah on its second radical; مَفْعِلٌ has kasrah there.
مَنْصَرٌ — mafʿal.
مَضْرِبٌ — mafʿil.
Both begin with a fatḥah-bearing mīm and retain a sukūn on the first radical. The second radical distinguishes the two patterns, using the imperfect form as the guide.
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Two common triliteral stem patterns
Ism al-Ẓarf: Meaning, Patterns, Paradigm and Derivation
Simple triliteral place/time nouns do not all share the same middle vowel. مَفْعَلٌ has fatḥah on its second radical; مَفْعِلٌ has kasrah there.
مَنْصَرٌ — mafʿal.
مَضْرِبٌ — mafʿil.
Both begin with a fatḥah-bearing mīm and retain a sukūn on the first radical. The second radical distinguishes the two patterns, using the imperfect form as the guide.