Why שלו is in the middle of the sentence?
Possessives come after the word in Hebrew: "the-tail his". Read the notes.
"When you have a noun, an adjective and a possessive, the possessive comes after both the noun and the adjective:
הכלב החדש שלי
My new dog
not הכלב שלי החדש"
Sorry, I think I understand now.
It is: His tail is long and not "His long tail". It is an english problem to me :/.
Correct, but thanks for mentioning the notes, there is room for improvement there, it's not the most clear, I admit.
How about "זנבו ארך"? It was marked wrong, but I think it ought to work in formal speech.
Couldn't I say
"הזנב שלו הוא ארוך."
Where "הוא" emphasizes "is"?
You can, but it's less natural saying it like that.
shouldn't this have a copula in it? הוא
No. Copula is normally used when you have a sentence that compares two nouns, not a noun and an adjective. Adding it here would be less natural.