I may be wrong on this as I'm just a beginner myself, but I believe 'Du' is if you were talking to a single person and 'Ni' a group of people. They mean the same thing but they're situational. So it's the same with 'din' and 'er' e.t.c. (I may be completely wrong on this and I could just go to Google to find the right answer but it's nice practice to try and work things out myself haha)
Oh, yeah that makes sense. I think you might be right. I asked my dentist (she's Swedish) and she said that din (for a singular noun) is singular and dina is plural for a singular noun, etc... (It's SUPER hard to explain). So you were right. Like the Duolingo translation is "Dina boker" (I spelled it wrong) Which is "your" (singular) books (plural). It is kind of confusing. I probably messed up on my translation but I think that you starting to make me comprehend this matter more. And if you think about it, it isn't that hard to understand. It's just hard to explain.