What's the difference between "dein" and "eure?"
They both mean "your", yet it's incorrect if you use one rather than the other at times. What's the reason for this?
The difference is between the second-person singular and plural. "Dein" is as "du", in that it refers only to one person (second-person singular). "Euer" is from "ihr", which refers to a group to whom you are speaking (second-person plural). So, when you use "du", also use "dein"; when you use "ihr", also use "euer".
An English example might be: "Dein Essen" - "Your food." "Euer Essen" - "You guys' food."
YOu get confused because in English you use the same word to mean a person you are talking to, as well as a group of people you are talking to. Besides, you don't make the difference between formal and informal. For us spaniards, it's easier because we have "tú" (singular informal you), "usted" (singular formal you), "vosotros" (plural informal you) and "ustedes" (plural formal you). "dein" is singular informal (for "du") and eure is for plural informal (for ihr).
That's a good point, but we do have ways to clear up ambiguities that are not in the translations. We would not say /You have shirts/ if we meant you plural. It would be more like /All of you have shirts/, or /Each of you has a shirt/. Maybe Duo should learn /Ya'all have shirts/ to avoid the ambiguity without adding extra words. I'm only half joking..
Adding to the other comments there is one thing you need to pay attention to. The form of German possessive pronouns follow two "objects", the thing owned and the owner.
The base form follows the owner: My - Mein, Your (singular) - Dein, His - Sein, Her - Ihr...
The ending is determined by the object owned.
Male: Mein Tisch
Female: MeinE Katze
Neuter: Mein Haus
Plural: MeinE Finger
And when used as an object in a sentence they additionally follow the case declensions, of course.
What I really want to say here is: It is not only a difference in numerus (dein - singular, eure - plural) here but also in the object owned. "dein" can be (in nominative) for male or plural objects, "eure" for female or plural. So both can be used for a number of objects owned but couldn't be used for a single item owned.
Given the plural and singular forms of you, it is still a challenge for translating statements like /You have shirts/. Any one person can have shirts, or more than one person can each have a shirt. The same with /Your men are strong/, I think of an army, or work crew not a set of individual husbands. I guess we sometimes just have to memorizes phrases.