Τον χυμό is the accusative case of ο χυμός and is used when it's the object of the sentence.
It's because there is a rule for την,δεν,μην,στην that says that the -ν is omitted when it is not followed by κ,π,τ,ψ,ξ,γκ,γγ,μπ,ντ or a vowel. The rule has changed a few times in past years, and in the past, it applied also to τον. Now, that is not the case, and it's considered wrong to omit -ν in τον under any circumstances. But the course creators used the old version of the rule. Now that we are in beta, we cannot change the original greek sentences, but it's on our to-do list after we graduate from beta to add those missing -ν.
I appreciate the curve balls that DL Gk provides by sometimes having the nu with masc acc. and sometimes not, because the people often drop it but discussions from our moderators indicate that we'll see it with the nu in writing (in order not to confuse with neuter) and possibly sometimes in spoken Gk. I've read that Triandaphyllidis based his morphological principles to a great extent on folk songs and literature (Mackridge, The Modern Gk Language, 13), so it makes sense to me that rules change to reflect actual spoken usage today. But my question is whether there exists a group that makes these decisions akin to Real Academia Española (RAE) for Spanish? The latter has an impossible task, as the Spanish language is so diffuse in the many nations and regions that speak varieties of that language. RAE tries to provide some ground rules for normativity while also attempting to respect the fluid nature of the language.
There is the "Center of Greek Language" (Κέντρο Ελληνικής Γλώσσας). It's the organisation responsible for studying objectively the language's trends. But, grammar rules are defined by school grammar books which are written by linguists assigned by the Ministry of Education. The books are then examined and corrected by a high-level researcher of the Center of Greek Language and then published for use, so they are supposed to be objective. However, I think that each lingust assigned to write the official Greek grammar has their own opinions and views on the matter, and therefore the rules are not truly objective. But that's also a form of regulation, I guess. Truth is, such silly rules as the final -ν, are rules the Greeks couldn't care less about. The only incorrect thing on this particular matter is not to use the nu when needed. If you use it and its unnecessary, no one will care, except for grammar nazis, whose "undeniable opinions" change every 15 years, along with the school grammar books.