I'm starting to write all nouns in all languages with capital letters ... Am I the only one?
Even BEFORE I started learning German, I had a tendency to capitalize all nouns, at leat in my native English (not so much in the Spanish & French I learned in school) ;~D. However, I have also started learning Italian on Duolingo, and I have to stop myself from capitalizing the Italian nouns! Capitalizing nouns just always felt right to me.
Alright then, possessive pronouns...
The first thing you have to know is German possessive pronouns are declined according to case, gender and number of the object, the thing that's being "possessed". So, you have "seine Orange" (feminine), "sein Apfel" (masculin) and "seine Orangen" / "seine Äpfel" (plural).
my - mein(e)
your - dein(e)
formal your - Ihr(e)
his+its / her - sein(e) / ihr(e)
our - unser(e)
pl. your - euer (eure)
their - ihr(e)
(die) Orange is a feminine noun, so it is "seine Orange" (and so also "seine dritte Orange").
(der) Apfel, however, is masculine, so "his third apple" would "sein dritter Apfel".
(I think it is "dritter" in that masculine example. There seem to be different rules for how ordinals decline dependning on whether they have an article, and I don't know for sure how those rules apply to possessive pronouns)
This doesn't seem to make sense to me because the third orange is still a single orange, not multiple oranges. To contrast, it would be different to say "That is his third dozen Oranges."
Is this just a German thing?.... Because this is not grammatically correct in English, and I can think of numerous examples. It would be incorrect to say "These are his third beer."
I'm not sure where you're seeing seine translated as "it's".
I do see that if one hovers over seine, the hints provide three options for seine: his, its, one's. (NB: "it's" is not "its". At all.) So, the answer is to your question is: it's not.
So, to answer the question you didn't ask: "why does die Eule hint that 'its' is a valid translation of seine?" Because in German, just like in English, one can apply the masculine--or feminine--pronoun to a neuter object. The application of the feminine to neuter objects is more familiar to English speakers: "Have you seen pictures of the RMS Titantic?
She was a beautiful ship."
The pronoun "sein(e)" is the possessive for both masculine and neuter. It means both "his" and the possessive "its".
If Duo suggested the contraction "it's", with an apostrophe, that would be an error.
Also remember that inanimate objects can be masculine or feminine, so an "it" in English sometimes translates to a masculine or feminine pronoun in German.
"Orang" vs "Orange"
I've just noticed on this lesson that Duo has gone from saying the word "Orange" with the "e" pronounced (o-rang-e) to a silent "e" (o-rang).
I went to DictCC and found out that it can be either one. Apparently, "orange" is of old French origin and thus German accepts the French form (o-rang). But, since it is an old word and German is still a growing language, it also appears to be acceptable to say it the German way (o-rang-e), with the "e" at the end.
But, this definitely caught me a bit off guard.