Nominative vs Accusative
Greek marks a word’s job in the sentence by changing its form (its case), not just its position. The nominative is the subject (the one doing the action); the accusative is the direct object (the one the action lands on). The same noun therefore looks different as subject vs object.
The rule
Greek marks a word's job by changing its form (its case), not merely its position. The nominative is the subject; the accusative is the direct object. The same noun therefore looks different in each role - ο δάσκαλος (subject) versus τον δάσκαλο (object) - and you pick the case by what the noun is doing in the clause.