The textbook derives these forms from the active imperfect. First separate the imperfect prefix from the three original root letters.
In يَضْرِبُ, the initial يَ is the imperfect prefix, while ḍād, rāʾ and bāʾ are the root letters.
The imperfect prefix is removed and a mīm is supplied in these three patterns. In instrument nouns this mīm has kasrah. Further changes depend on the chosen pattern and the vowel of the second root letter.
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Starting from the imperfect
Ism al-Ālah: Definition, Three Patterns, Paradigms and Formation
The textbook derives these forms from the active imperfect. First separate the imperfect prefix from the three original root letters.
In يَضْرِبُ, the initial يَ is the imperfect prefix, while ḍād, rāʾ and bāʾ are the root letters.
The imperfect prefix is removed and a mīm is supplied in these three patterns. In instrument nouns this mīm has kasrah. Further changes depend on the chosen pattern and the vowel of the second root letter.