The context should help you distinguish "ist" and "isst".
The verb "sein" (ist) has a linking function, so it takes the Nominative "ein Apfel":
- Meine Enkelin ist ein Apfel. = My granddaughter is an apple.
"An apple" is in the Akkusativ form "einen Apfel" because the apple receives the action:
- Meine Enkelin isst einen Apfel. = My granddaughter eats (is eating) an apple.
Lose that thinking and you'll have more fun learning German. Discovering that another language has a unique word for something that you'd need to describe or use a compound word for in your own language can be enlightening.
English uses "niece" and "nephew", but maybe some other language just calls them both "sibling-child" or something.
I enjoyed learning that there's a single word for "leaf litter" in German: Laub