Dutch doesn't have a case system (anymore).
The following is about the singular you. Jij is subject, jou is object and jouw indicates possession. Next to that there is van jou which is similar to English yours, this also indicates possession:
- jij praat = you talk (jij is subject)
- ik praat met jou = I talk with you (jou is object)
- ik hou van jou = I love you (jou is object)
- het is jouw boek = it is your book (jouw is a possessive adjective)
- het boek is van jou = the book is yours (van jou is a possessive pronoun)
Jij, jou and jouw can all be replaced by je (the first 4 examples), however this is not the case when using the possessive van jou (last example).
Also see this explanation: https://www.duolingo.com/comment/3732923
:) You're right that you can use it that way. But in that sentence jou is the object (similar to my second and third example), it's not a possessive. I mentioned you (usually) cannot replace van jou by van je when this is the possessive (like in my last example). I'll add some italics to make it more clear in my previous post.
So there is no exception here.