"Stephanus goes to the market with Corinna."
Translation:Stephanus cum Corinna ad forum it.
In the original English sentence, "Stephanus" is the subject, "goes" is the verb; "to the market" and "with Corinna" are both prepositional phrases... in this instance, adverb(ial) prepositional phrases. No matter how many people are with Stephanus, he is the single subject, and, so, the verb must be 3rd person singular.
As i understand it, it's because only one of the people named are the object, the other is mentioned as an aside. 'It' therefore works for the singular, in the same way that in English you might say "Mark goes to the market with Katie" rather than "Mark going to the market with Katie".