"You do not have an apple."
Translation:Je hebt geen appel.
Geen is used to negate a noun that, if not negated, would be preceded by een. You can say that geen translates to ‘not a’.
Geen is also used if the noun is not preceded by any article, like some plural and uncountable nouns.
Niet is essentially used in all other situations
You have no apples. In english you use the plural form of the verb with a count of zero.