"Your oranges are big."
Translation:Eure Orangen sind groß.
Deine Orangen sind groß is also a possible translation of this sentence and should already be accepted.
Because it's the subject, so it's in the nominative case -- thus the ending for plural is -e.
-en would be in the dative case.
For ease of pronunciation, I suppose.
There are a few cases where -er, -el, -en endings lose their -e- when a vowel is added to the end.
Some more examples: "expensive" is teuer but "more expensive" is teurer (not teuerer); "to wrap" is wickeln but "I wrap" is ich wickle (not ich wickele); "dry" is trocken but "to dry (something)" is trocknen (not trockenen).
"Orangen" is plural, thus an "e" is added to the end of "Eure" to match the Noun in the sentence.
I believe I'm forgetting something I learned long time ago. Why is it "groß" and not "große" again?
Because it's a predicative adjective -- it's after the verb "to be" rather than before a noun.
Predicative adjectives don't have endings for gender, number, or case in German, so it's just groß without any ending.
Der Mann ist groß. Die Frau ist groß. Das Kind ist groß. Alle Menschen sind groß.
They're both accepted, since the English "you" is ambiguous.
- deine = informal singular
- eure = informal plural
- Ihre = formal