Every sentence in this lesson puts the noun in the dative case — after a dative preposition (mit, aus, seit, von, zu — "with/from/since/of/to"), or as the person being helped, thanked, believed, answered, etc. (helfen, danken, folgen, gehören, glauben, gratulieren, antworten, zuhören, schmecken, passen, begegnen all take a dative object). The indefinite article changes depending on the noun's gender:

Masculine and neuter share the same dative article, einem — only the feminine article changes, from eine to einer. This is the opposite pattern from the accusative case, where masculine was the one that changed.