A noun used in isolation (i.e. not as part of a sentence) is assumed to be in the nominative case, e.g.

Ein toller Film, nicht?
A great i lm, don’t you think?

If an isolated noun is in fact the object of an otherwise unuttered sentence, as in abbreviated answers to questions, the accusative or dative may be required, e.g.

A: Was liest du? B: Einen Roman.

A: What are you reading. B: A novel.

A: Wem hast du das Geld gegeben?
B: Dem Sohn.

A: Who did you give the money to?
B: The son.

People are addressed in the nominative case, e.g.

Was ist passiert, mein lieber Freund?
What (has) happened, my dear friend?

Complements of the following so-called copula verbs are in the nominative case; copula verbs take the nom. case both before and after them: bleiben (to remain, stay), scheinen (to seem, appear), sein (to be) and werden (to become), e.g.

Er ist ein sehr guter Lehrer.

He is a very good teacher.

Er wird ein ausgezeichneter Athlet.

He’s becoming an excellent athlete.

Er scheint ein ausgezeichneter Athlet zu sein.

He seems to be an excellent athlete.