No rule for this one
No ending or word group predicts the article of Morgen. Learn it as a pair and say it out loud: der Morgen.
der Morgen
It's der Morgen. Morgen is masculine, plural die Morgen.
Am Morgen trinke ich Tee.
In the morning I drink tea.
No ending or word group predicts the article of Morgen. Learn it as a pair and say it out loud: der Morgen.