Yes. There is a difference. 1. 'I ate an apple' tells you something about a time in the past. For example: "What did you do when you got to the top of the hill?" "I ate an apple." 2. 'I've eaten an apple' tells you something about the present time. For example: "Are you hungry? Have you had anything to eat today?" "Yes, I've eaten an apple." This explains why you aren't hungry now. "There's some fruit missing. Have taken any fruit from this fruit bowl?" "Yes, I've eaten an apple." This explains why there is some fruit missing from the bowl. I hope that helps.