When the noun is the subject of the sentence, you use the nominative case pies. When the noun is the object following the verb "to be" (być), as in "...is a dog," you use the instrumental case "...jest psem." Except when the phrase "to jest" or the word "to" is used. Then, the noun stays in the nominative. "To jest pies." "To pies."
Is there a website similar to the one below but for Polish instead of Russian?