The first textbook pattern, مِفْعَلٌ, runs:
Word / formMeaning / explanation
مِفْعَلٌsingular, one instrument.
مِفْعَلَانِdual, two instruments.
مَفَاعِلُbroken plural, several instruments.
With the root ḍ-r-b the forms are مِضْرَبٌ، مِضْرَبَانِ، مَضَارِبُ. The plural changes the internal pattern, including a new alif, rather than simply appending -āt or -ūn.
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Ism al-Ālah: Definition, Three Patterns, Paradigms and Formation
The first textbook pattern, مِفْعَلٌ, runs:
Word / formMeaning / explanation
مِفْعَلٌsingular, one instrument.
مِفْعَلَانِdual, two instruments.
مَفَاعِلُbroken plural, several instruments.
With the root ḍ-r-b the forms are مِضْرَبٌ، مِضْرَبَانِ، مَضَارِبُ. The plural changes the internal pattern, including a new alif, rather than simply appending -āt or -ūn.