It can be "an apple" -- that's okay. (Or it can be "apples," because it doesn't need the plural ending if it's a general direct object.)
It can't be "the women eat apple," because that's a problem in English. (In English, you can have "an apple" or "the apple" or "apples," but you can't have just "apple.")
If I've understood turkish logic, the plural form is not used because not necessary for distinction with direct object complement.
- "I eat the apple" is "elmayı yerim" (singular without defined article -> accusative case)
- "I eat the apples" is "elmaları yerim" ((plural without defined article -> plural mark "lar" mandatory + accusative case)
- "I eat one apple" is "bir elma yerim" (singular with undefined article "bir", accusative case not applied with indefined form)
- "I eat apples" is "elma yerim" (singular without undefined article "bir" article, thus it is plural and "lar" suffix is omitted, accusative case not applied with indefined form)
I hope to be understandable; I'm sorry, but I don't speak English. I am self-taught.