"He sees a big castle and a big bear."
Translation:On vidí velký hrad a velkého medvěda.
As someone who is not a native Czech speaker, I feel that I should explain here that both "hrad" and "medvěd" are masculine nouns, but that the reason that the adjectives modifying them are different in the accusative case ("velký" as opposed to "velkého") is due to a distinction between what is animate and what is inanimate. Velkého describes medvěda because bears are living things. Castles are inanimate, so they just use velký. I lived for a year in Olomouc when I was 19, and I remember this being a challenging aspect of an already difficult language.