Possessive pronoun and adjectives

This pen belongs to me
(a) It’s mine
(b) It is my pen
(a) And (b) have the same meaning: they both show possession. Mine is possessive pronoun; my is a possessive adjective.
     Possessive pronoun      Possessive adjective
(c) I have mine.                  I have my pen
(d) You have yours.            You have your pen
(e) She has hers.                  She has her pen
(f) He has his.                      He has his pen
(g) We have ours.                We have our pen
(h) They have theirs.             They have their pens
(i)                                       I have a book. Its cover is black.
A possessive pronoun is used alone, without a noun following it
A possessive adjective is used only with a noun following it:
Incorrect: I have mine pen
Incorrect: I have my
Compare its vs. it’s
(j) Sue gave me a book. I don’t remember its title
(k) Sue gave me a book. It’s a novel
In (j): its (NO apostrophe) is a possessive adjective modifying the noun title
In (k): it’s (with an apostrophe) is a contraction of it + is.
Compare their vs. there vs., they’re
(l) The students have their books
(m) My books are over there
(n) Where are the students? They’re in the class
Their, there, they’re have the same pronunciation, but not the same meaning.
Their = possessive adjective, as in (l)
There = an expression of place, as in (m)
They’re = they are, as in (n)