How do we know who is visiting who? So far Latin seems to be more about getting meaning from grammar rather than word order, but aside from order, I can't figure out how I would know.
Stephanus Marcum visitat - Stephanus visits Marcus
Stephanum Marcus visitat - Marcus visits Stephanus
See? It is the cases that determine who does what to whom, etc.
The -us ending here shows who is the subject, the 'doer' of the verb, and the -um shows who is the direct object of the verb. The first is called the nominative case, the other the accusative. More on this in the Tips and Notes.
Otherwise, any word order would work in this sentence, rendering the same meaning.