When should I use the nominative case?

The nominative case is used for the subject of the verb, i.e. the noun doing the action. Consider this example: my brother has a dog. In this simple sentence, my brother is the subject of the verb as he owns the dog - mein Bruder hat einen Hund. The nominative case is also used with the verb 'sein', for example Mr. Mann is a good friend - Mr. Mann ist ein guter Freund. This is because Mr. Mann and the good friend are the same, hence the nominative case is used for both. Finally, the nominative case is used with nouns used in isolation, for example ein schöner Tag!