In this rule the penultimate letter carries kasrah. If the active imperfect already has that kasrah it remains; otherwise the required kasrah is supplied.
Word / formMeaning / explanation
يُكْرِمُ ← مُكْرِمٌkasrah on rāʾ.
يَجْتَنِبُ ← مُجْتَنِبٌkasrah on nūn.
يَتَقَبَّلُ ← مُتَقَبِّلٌkasrah on the doubled bāʾ.
Simply adding initial mu- is therefore insufficient unless the internal vowel is correct.