How do I know when I have to use Eure and when to use dein?
You use "dein" when referring to a single person, e.g. "dein Buch - your book".
You use "eure" when referring to a group, e.g. "eure Tassen - your cups"
Further examples: euer Buch: talking to the coauthors of a book, eure Tasse: talking to the couple drinking from one cup, deine Tasse: talking to one person with one cup