"You drink a man's juice."
Translation:I drikker en mands saft.
i is the plural you, so this sentence is kind of awkward because it is multiple "yous" drinking the man's juice, not just one person.
(hopefully someone will correct me if I am wrong)
They are usually interchangable. The only difference is that you would use "Du" when you are talking to a single person ("you are happy"), and you would use "I" when talking to more than one person ("you all are happy").
I think it is similar to French (tu/vous) and Spanish (tu/usted).
You are correct, just I wouldn't describe them as interchangeable, unless you mean interchangeable with the English "you"
In Spanish that kind of grammar rule would be tú/ustedes.
You are a happy person=Tú eres una persona felíz. You all are happy=Todos ustedes son felices. "Usted" is a formals way to say "Tú", is like the most polite way to talk to someone. So, "Ustedes" is the plural form of "Tú/Usted".
Hope this helps.
Nothing wrong with the sentence! ... Unless you're reading something else into it ;-)