I've seen the danish word "jo" used often and I guess it's supposed to mean "the" but I am not clear on why/how/when?
We do not teach jo in the course, because even though it can be used as an adverb meaning obviously, then the most common usage of jo comes from it's response to questions, and a certain kind of question.
In English you always respond with yes or no to a question, but in Danish you have three ways of responding to a question, depending on how the question is phrased. You have the two yes forms: ja and jo, and then no which is nej. Jo is only ever used with negated questions: Kommer du ikke i morgen? (Are you not coming tomorrow), which you'd say yes to in the following way: Jo (usually followed by some reasoning). Whereas Ja would be used for questions such as Kommer du i morgen? (Are you coming tomorrow).
Therefore we have not introduced jo in the course, since it requires a sentence before it.
I hope you would consider adding jo in future as it's a pretty fundamental part of the language when used as a reply to a negative question. In French, the affirmative answer to a negative question is "si" which works like jo in this context. It might be worth looking at how and if they teach si in the French course.
Jo is a funny word, it can mean yes when the question contains a negative, but other than that it's not really got a very good English translation. It's used to kind of like "of course/obviously". Example I found here has the example: "Selv om de er rare at have, er penge jo ikke alt her i livet", which would translate as "Even though it's nice to have, money obviously isn't everything". (Please note I'm not a native or very proficient, jo probably also has other meanings, but these are the main ones I can think of, I've never known it meaning "the")
As the previous comment says, jo is a funny word. I'm a native Dane, and it's pretty much all over the place. It can be a sarcastic "yes" - You have a roof over your head, so why do you complain? (Du har tag over dit hoved, så hvorfor brokker du dig?) - Yes, but (Jo, men) It can be used as "of course" depending on what you want to say - Don't you love me? (Elsker du mig ikke?) - Of course! (Jo!) I've also heard someone describe the word "jo" as "Expressing that something is obvious to all and expression of a concession", if that makes sense. But it doesn't take the place for a "the". That's what en/et, den/det is for.