No rule for this one
No ending or word group predicts the article of Schlüssel. Learn it as a pair and say it out loud: der Schlüssel.
der Schlüssel
It's der Schlüssel. Schlüssel is masculine, plural die Schlüssel.
Wo ist der Schlüssel?
Where is the key?
Hier ist der Schlüssel.
Here is the key.
Ich habe den Schlüssel verloren.
I have lost the key.
No ending or word group predicts the article of Schlüssel. Learn it as a pair and say it out loud: der Schlüssel.