"Marcus comes from Germany."
Translation:Marcus a Germania venit.
Marcus is nominative, used as the subject of a sentence, if Marcus is doing an action. Used here since he is the one 'coming'.
Marcus librum habet (Marcus has a book). Marcus is 'doing' the having.
Marcus is accusative, usually the direct object of the verb, used if Marcus has some action done to him.
Livia Marcum videt (Livia sees Marcus). Marcus is the one being seen.