"They drink the juice."
Translation:Αυτοί πίνουν τον χυμό.
It's because χυμός is masculine.
According to the current rules, the masculine accusative article is always τον with a final nu.
(When I learned Greek, the rules were different and it would have been το in this sentence; the rules then took into account the first sound of the following word in deciding whether to use το or τον.)
I appreciate the clarification that the final nu is the current rule and that you grew up with it being dropped. To drop or not to drop? I've read elsewhere that the rule was adopted (I'm not sure when or what body made the rule) to help avoid confusion with neuter nouns but that the people routinely drop the nu, on the one hand, and that formal Gk will keep the nu no matter what letter follows.