The six triliteral unaugmented pairs are:
فَعَلَ يَفْعُلُ — نَصَرَ يَنْصُرُ
فَعَلَ يَفْعِلُ — ضَرَبَ يَضْرِبُ
فَعِلَ يَفْعَلُ — سَمِعَ يَسْمَعُ
فَعَلَ يَفْعَلُ — فَتَحَ يَفْتَحُ
فَعُلَ يَفْعُلُ — كَرُمَ يَكْرُمُ
فَعِلَ يَفْعِلُ — حَسِبَ يَحْسِبُ
Each imperfect belongs with the preceding perfect as an attested pair. Mixing the vowels of unrelated forms creates an invalid class assignment.
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All six patterns in one comparison
Verb Classes and the Six Unaugmented Triliteral Patterns
The six triliteral unaugmented pairs are:
فَعَلَ يَفْعُلُ — نَصَرَ يَنْصُرُ
فَعَلَ يَفْعِلُ — ضَرَبَ يَضْرِبُ
فَعِلَ يَفْعَلُ — سَمِعَ يَسْمَعُ
فَعَلَ يَفْعَلُ — فَتَحَ يَفْتَحُ
فَعُلَ يَفْعُلُ — كَرُمَ يَكْرُمُ
فَعِلَ يَفْعِلُ — حَسِبَ يَحْسِبُ
Each imperfect belongs with the preceding perfect as an attested pair. Mixing the vowels of unrelated forms creates an invalid class assignment.