Actor in an event who initiates and carries out the event intentionally or consciously, and who exists independently of the event.
THEY decided to go.
Undergoer that is central to an event or state that does not have control over the way the event occurs, is not structurally changed by the event, and/or is characterized as being in a certain position or condition throughout the state.
THE BALL rolled down the hill.
Undergoer in a state or an event that is (potentially) advantaged or disadvantaged by the event or state.
Martha carved a toy FOR THE BABY.