"A lion is a cat."
Translation:Lew jest kotem.
"Kot" is Nominative, "kotem" is Instrumental. There are two ways to translate a sentence like "X is Y" when both X and Y are noun phrases (you will soon find explanations under some other sentences). One is "X + a form of 'być' + Y" in Instrumental (so: Lew jest kotem), and the other is simple "X to Y" with Y in Nominative, like here.