Der Hund läuft.
English: The dog runs.
The Nominativ is the basic form of nouns and pronouns in German. It is used for the subject of a sentence—the person or thing doing the action.
Use the Nominativ for the subject of a sentence, after the verb 'sein' (to be), and for nouns and pronouns that are doing the action.
Der Hund läuft.
English: The dog runs.
Die Katze schläft.
English: The cat sleeps.
Das Buch ist neu.
English: The book is new.
Die Kinder spielen.
English: The children play.