The subject of a sentence is always in the nominative.

Der Mann kommt aus Frankreich.

Sometimes there is also a second nominative in the sentence, e.g. with the verb sein:

Der Mann ist ein Feinschmecker.

The second nominative gives more detail about the subject. It can only be used with a few verbs, e.g., werden and bleiben as well as sein. These verbs link the subject to a noun in the nominative.

Nominative articles

Here is another overview of some nominative articles that you have already studied.