Learn Polish: Nazwała go bohaterem. - She called him a hero.

Verbs of naming, calling, and electing, such as nazwać, trigger a specific case pattern. The person or thing receiving the name takes the Accusative (here, go), and the label or title applied to them takes the Instrumental (here, bohaterem).

Why use the short pronoun go instead of jego?

Go is the unstressed form of the Accusative pronoun for 'him'. Since there is no special emphasis on 'him' in this sentence, the short form is standard. If you wanted to contrast him with someone else (She called him a hero, not the other guy), you would use the stressed form jego.

What would this sentence mean if we used the imperfective verb nazywała instead of nazwała?

The perfective nazwała shows this was a single, completed event—she called him a hero on one specific occasion. If you used the imperfective nazywała, it would mean she used to call him a hero repeatedly over time, or was in the habit of doing so.

Could this sentence also mean 'She named him Bohater' (as if it were his actual given name)?

Yes, theoretically. The structure nazwać + Accusative + Instrumental can be used for giving actual names (like naming a child or a pet) as well as for assigning descriptive labels. Context is what tells you whether 'Bohater' is meant as a literal proper name or a descriptive title.