an apple 只一个苹果
eg：I want an apple for my birthday.我想要一个苹果作为生日礼物.
the apple 特指某个苹果
eg:There is an big apple,I like the apple.这有一个大苹果,我喜欢这个大苹果.
apples 泛指 苹果 I like apples.我喜欢吃苹果.
the apples 特指某些苹果
There are so many green apples,and I like the apples.这有很多绿苹果,我喜欢这些苹果.