"A dutiful man sacrifices."
Translation:Vir pius sacrificat.
You use pium when the case is accusative, where the modified noun is an object, e.g. mater filium pium habet, meaning a mother (subject) has a dutiful son (object) --- who does the mother have? A dutiful son.
And you'd use pius for nominative case, where the modified noun is a subject, e.g. filius (subject) pius est, or ubi est filius (subject) pius? --- who is dutiful? The son is.
Anyone please correct me if I'm wrong.
Please, read the basics about cases in Latin, before going further in the course:
(Just focus only on nominative and accusative for now):