The Accusative of the Article and Noun
When a noun phrase becomes the direct object, BOTH the article and the noun shift to the accusative together: ο δάσκαλος (subject) → τον δάσκαλο (object). The article and noun always agree, so they change as a unit.
The rule
When a noun phrase is the direct object, both the article and the noun move to the accusative together: ο δάσκαλος → τον δάσκαλο. They always agree, so treat the article+noun as a single unit that shifts case as one.