The two participles look very similar, so compare them directly.
MeaningForm
one who avoidsمُجْتَنِبٌ
that which is avoidedمُجْتَنَبٌ
The visible difference is in the middle vowel: مُجْتَنِبٌ is active, while مُجْتَنَبٌ is passive.
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Ism Fa'il and Ism Maf'ool
Derived Trilateral (with Hamzah) - Bab 1: Ifti'aal (Ijtinab)
The two participles look very similar, so compare them directly.
MeaningForm
one who avoidsمُجْتَنِبٌ
that which is avoidedمُجْتَنَبٌ
The visible difference is in the middle vowel: مُجْتَنِبٌ is active, while مُجْتَنَبٌ is passive.