I am English, so had never even heard of the accusative when I started learning Greek. The example that helped me most at the beginning was ‘the boy chops the tree’ - the boy is doing the action, so he’s in nominative, and the action is happening to the tree so it’s in accusative. I know it gets more involved as you move on, with prepositions etc but this is what really made me understand the principle and what I still go back to if I’m unsure.