"Do you have a cup of coffee?"
Translation:Har du en kopp kaffe?
Here in the Southern U.S., we often say "y'all " which is short for "you all". Think of du as you and dere as y'all, with no slang connotations. So, du is just the person in front of you, but dere is that person plus his/her associates OR everyone in the room OR the rest of the planet or whatever. I find it very useful, especially when asking about food. Good luck & enjoy learning more