Explanation
The possessive adjectives are my, your, his, her, its, our, and their.
The possessive pronouns are mine, yours, his, hers, ours, and theirs. There is no possessive pronoun for it.
Possessive adjectives are always used with a noun or noun phrase. They do not occur alone: my book, your car, his pen, her shoes, etc.
Possessive pronouns (mine, yours, his, hers, ours, its, and theirs) function alone as nouns.
my car-->mine
your car-->yours
his car-->his
her car-->hers
our car-->ours
their car-->theirs
That is my car. It is mine.
Incorrect-->That car is my.
Your cup is in the cabinet. Yours is in the cabinet.
Incorrect-->Your is in the cabinet.
That is his cell phone. It is his.
These are her shoes. They are hers.
Incorrect-->They are her.
etc.
There is no change in the possessive adjective if the noun is plural.
my books-->mine
your pens-->yours
his shoes-->his
etc.
Those shoes are hers.