نَصَرَ means “he helped,” reporting a past event; نَاصِرٌ means “one who helps,” identifying an agent.
Word / formMeaning / explanation
ضَرَبَhe struck.
ضَارِبٌone who strikes.
سَمِعَhe heard.
سَامِعٌone who hears.
The active participle is thus a derived noun, not merely an additional past or imperfect verb form.