Video narration
Why does der Hund suddenly become den Hund? It can feel random. But use the English sentence, "I see the dog." The dog is the thing receiving the action. German treats it as the direct object and marks it with the Akkusativ, the case used here. The change is not random. Ask: who or what receives the action? In "Ich sehe den Hund," Hund is masculine, so der becomes den. With an indefinite article, ein becomes einen: "Ich kaufe einen Tisch." "Sie hat den Ball"; the ball receives the action. "Wir sehen die Katze"-cat is feminine, and die stays die. In this basic pattern, feminine die, neuter das, and plural die usually stay unchanged. The article gives the visible clue, not the noun. So in "I see the dog," German gives you den Hund. Find the object, then choose the accusative article.