For the regular sound three-radical perfect verb, the active participle follows فَاعِلٌ: an alif appears after the first radical and the second radical takes kasrah.
Word / formMeaning / explanation
نَصَرَ ← نَاصِرٌhelper.
ضَرَبَ ← ضَارِبٌstriker.
سَمِعَ ← سَامِعٌlistener.
The letters fāʾ, ʿayn and lām in the pattern are placeholders for the actual root consonants.