"We visit the orator in the forum."
Translation:Oratorem in foro visitamus.
Ōrātor is equivalent to "HE, the orator."
You wouldn't say, "We will visit HE in the forum."
Instead, you need the form for "orator" that can be used as the direct object of the verb "to visit," namely, ōrātōrem , which is equivalent to "HIM, the orator."
You can't just use one form of a noun 'over and over,' in Latin! Each noun has five different singular "cases," or forms, and five plural ones.
For example: "The orator is delivering a speech." (the orator is a HE: he's doing something: he's the subject of the verb "is delivering." In Latin terms, we're using the nominative case, for "orator.") Ōrātor ōrātiōnem habet .
Or, "The orator's speech pleases me." (the orator's: he's a possessive; he's only 'in the sentence' as the owner of the speech. It's HIS speech. In Latin terms: genitive case) Ōrātōris ōrātiō mihi placet .
Or, "The crowd supports the orator." / "The crowd gives support to the orator." (this particular verb requires the dative case or indirect object function: to/for him) Multitūdō ōrātōrī favet .
The Duolingo sentence illustrates the accusative case (direct object function).
"We are speaking with the orator." (Preposition "with" in Latin requires the ablative case.) Cum ōrātōre loquimur .
Bear in mind that five different forms are needed, for plural "orators" in all of these separate functions.