"How is Stephanus?"
Translation:Quid agit Stephanus?
I'm no expert so don't take my word for this, but the answer seems to be yes. That is what Duolingo is telling us. I don't know whether or not this was a common phrase in Latin
The two most obvious words are 'agent' - 'he who does' and 'action' - the thing that is done. Why action, you might wonder? It is sometimes necessary to look at other parts of the verb, especially the supine. The four essential parts to learn are: the first person singular present, the present infinitive, the first person singular perfect, the supine, masculine form This gives us: ago, agere, egi, actum This is how the verbs were drilled into us as schoolchildren. Very useful!
Stephanus is not being spoken to (vocative), but about. This would normally be accusative, however as the verb is ESSE (to be) we use the copula, (linking subject with subject complement) keeping Stephanus nominative.