The accusative case

German marks the role a noun plays in a sentence by changing its article. The subject (who's doing it) is in the nominative: der Hund. The direct object (what it's done to) is in the accusative — and for masculine nouns only, der becomes den and ein becomes einen.

Good news: feminine, neuter, and plural don't change at all in the accusative. Ich sehe die Frau, das Kind, die Kinder. Only the masculine moves.