For the third pattern, مِفْعَالٌ, replace the active imperfect prefix with مِ and insert an alif after the second root letter. Complete the noun with its citation-form ending.
With ḍ-r-b:
مِضْرَبٌ — first instrument pattern.
مِضْرَابٌ — third instrument pattern.
Both retain the root ḍ-r-b. The extra alif after rāʾ is what identifies the longer مِفْعَالٌ formation.
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Third rule: adding an alif after ʿayn
Ism al-Ālah: Definition, Three Patterns, Paradigms and Formation
For the third pattern, مِفْعَالٌ, replace the active imperfect prefix with مِ and insert an alif after the second root letter. Complete the noun with its citation-form ending.
With ḍ-r-b:
مِضْرَبٌ — first instrument pattern.
مِضْرَابٌ — third instrument pattern.
Both retain the root ḍ-r-b. The extra alif after rāʾ is what identifies the longer مِفْعَالٌ formation.