"My" se pojí se jménem, "Mine" jako přivlastňovací zájmeno stojící samostatně.
Příklad: "This is my house." vs "This house is mine."
in an earlier exercise, my book was má kniha. now it's saying I have to use moje in this sentence, where the pronoun comes after the noun being possessed. Does position of the pronoun have something to do with it or is another factor playing a role?
They both should be perfectly interchangable and you actually could have reported it here :-)
The only problem with the sentence "Ta kniha je má" is that the "má" could be understood as verb "has" and "je" as "them". Therefore, without context, "The book has them" is also a valid translation of the sentence "Ta kniha je má." And that's the reason why it's still missing and "moje" sounds much better here though "má" is also ok.
The system accepts "Ta kniha je má", but it sounds rather unnatural. Officially the two forms are interchangeable, but in reality they aren't. In most situations one form or the other sounds better, but there aren't any clear rules, unlike English, where it's very easy to explain why it's "my book" but "the book is mine".