No rule for this one
No ending or word group predicts the article of Garten. Learn it as a pair and say it out loud: der Garten.
der Garten
It's der Garten. Garten is masculine, plural die Gärten.
Der Garten ist klein.
The garden is small.
No ending or word group predicts the article of Garten. Learn it as a pair and say it out loud: der Garten.