No rule for this one
No ending or word group predicts the article of Sofa. Learn it as a pair and say it out loud: das Sofa.
das Sofa
It's das Sofa. Sofa is neuter, plural die Sofas.
Das Sofa ist bequem.
The sofa is comfortable.
No ending or word group predicts the article of Sofa. Learn it as a pair and say it out loud: das Sofa.