Every action has a beginning, a middle, and an end.
We just know that at a certain time in the past, he was in the middle of eating. This focus on the middle of an action is called the imperfective aspect . In each case, there is no notion that the action began or ended, only that at some point it was on-going. If the imperfect is used to denote the middle of an action, the preterite is used to indicate the beginning or the end of an action. Sometimes it requires some thought to determine which part of the action is being described.