Ok, I think I'm needing some help here. So Dein means your when I'm talking to one person and one object "Dein Junge ist schnell." If I am talking to several people possessing several objects I should use Eure "Eure Jungen sind schnell". What if I am talking to several people with one same object? If I am talking to the parents about their boy, should it be "Eur Junge ist schnell"? What if I am talking to one parent with several boys, should it be "Deine Jungen sind schnell"?
No prob :) What you wrote there is nearly correct, but you sound a bit confused. Whether you're talking to one or multiple people determines whether you should use 'dein' or 'euer' (same as with du and ihr). The object has nothing to do with choosing dein or euer. What the object affects is the ending of the word - eg dein/deine/deinen. And its the gender of the object that determines the ending - masculine, feminine, neuter or plural (think of plural as a fourth gender). Let me give you some examples.
Talking to one person (du)
Dein Junge ist schnell - object is singular maskuline
Deine Katze ist schnell - object is singular feminine
Dein Baby ist schnell - object is singular neuter
Deine Jungen sind schnell - object is plural
Likewise talking to several people that you are familiar with (ihr)
Euer Junge ist schnell - object is singular maskuline
Eure Katze ist schnell - object is singular feminine
Euer Baby ist schnell - object is singular neuter
Eure Jungen sind schnell - object is plural
And talking to one or multiple people that you are not familiar with (Sie)
Ihr Junge ist schnell - object is singular maskuline
Ihre Katze ist schnell - object is singular feminine
Ihr Baby ist schnell - object is singular neuter
Ihre Jungen sind schnell - object is plural
Note that 'euer' is irregular - 'euer' is the base form, and when it takes an ending (eg. -e), instead of becoming 'euere', it is 'eure'.
And to confuse this further, the sentence case also affects the ending, here everything is in the nominative case. But I don't want to give you information overload, all the tables of endings are on wikipedia, see the first two tables under 'indefinite articles'. http://en.wikipedia.org/wiki/German_declension#Indefinite_articles The endings are the same for indefinite articles (ein, kein) and possessive adjectives (dein, eure, sein etc).
Thank you very much for this very long and very good answer. Duolingo lacks some explanations about this part and learning by just checking the correct translation is complicated since because English is too simple (just think about the amount of things that can translate to "your"..).
Is there any specially good reference I should consult in general, with all the tables and so?
Thank you again :)
I think this means eurer is more common than euerer with the extra E in the middle.
As with all possessive determiners, euer will take different endings depending on the case, gender and plurality of the noun. With other determiners, the root does not change and the ending is simply added to the root, but with euer it is normal to drop the second E when adding the ending so it becomes eur- plus ending.
For dative and genitive feminine nouns and genitive plural nouns, the ending is -er, yielding eurer, rather than than the relatively rare euerer.
Coming back to the translation of Your boys are fast, this should use the nominative (because it is the subject of the sentence) plural form - eure.
The less common spelling would be euere, but this is almost never used.