Countable and Uncountable Nouns Activities

Hello. This is a great question.
Apples are countable and therefore when quantifying them, you will use the plural form, except if just one is being referred to. Therefore, 'I bought some apples' but 'I bought an apple'.
With uncountable nouns, the form of the noun doesn't change - 'I bought some milk' and 'I bought milk'. 

I hope that this helps you.