Take سَمِعَ as an example:
• First letter: سَ — Fathah
• Middle letter: مِ — Kasrah
• Last letter: عَ — Fathah
Because the middle letter has Kasrah, its pattern is فَعِلَ.
Now compare: ضَرَبَ has Fathah in the middle, so it is on فَعَلَ.
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How to Recognize the Pattern
Foundations of the Past Tense Verb (Fi'l Maadhi)
Take سَمِعَ as an example:
• First letter: سَ — Fathah
• Middle letter: مِ — Kasrah
• Last letter: عَ — Fathah
Because the middle letter has Kasrah, its pattern is فَعِلَ.
Now compare: ضَرَبَ has Fathah in the middle, so it is on فَعَلَ.