No rule for this one
No ending or word group predicts the article of Nachbar. Learn it as a pair and say it out loud: der Nachbar.
der Nachbar
It's der Nachbar. Nachbar is masculine, plural die Nachbarn.
Mein Nachbar ist sehr nett.
My neighbour is very nice.
Mein Nachbar ist freundlich.
My neighbour is friendly.
No ending or word group predicts the article of Nachbar. Learn it as a pair and say it out loud: der Nachbar.