Possessive pronouns (mine, yours, his, hers, its, ours, theirs) show ownership and stand alone as the complete predicate or object, without a following noun. They differ from possessive determiners (my, your, his, her, its, our, their) which must come before a noun. For example, 'That book is mine' uses the possessive pronoun 'mine', while 'That is my book' uses the possessive determiner 'my'.
From your work with pronouns, you know that pronouns stand in for nouns to avoid repetition. Possessive pronouns do this for possession — they replace an entire possessive noun phrase. Instead of "This is Sarah's book and that is Marcus's book," you can say "This is Sarah's and that is Marcus's" — or, when the owner is already understood, just "This is mine and that is his." The pronoun swallows both the owner and the thing owned into a single compact word.
The key distinction to master is between the possessive pronoun and the possessive determiner. They look nearly identical — "my" and "mine," "your" and "yours," "their" and "theirs" — and they express the same relationship (ownership), but they function differently in a sentence. A possessive determiner is a modifier: it always appears directly before a noun. "My car is old" — "my" modifies "car" and cannot stand alone. A possessive pronoun is an independent word that takes the place of the whole noun phrase. "Mine is old" — "mine" replaces "my car" entirely. You could not say "My is old."
The test is simple: try removing the noun. If the sentence still makes sense without the noun, you need the possessive pronoun form. "That car is mine" — remove the implied noun ("my car") and "mine" stands perfectly on its own as the subject complement. If you said "That car is my," the sentence breaks — "my" needs a noun to hold onto. This is why possessive pronouns are sometimes called absolute possessives: they don't need anything else to complete their meaning.
Notice that "his" and "its" are the same in both forms — "his book / that is his," "its lid / that is its" (though "its" as a standalone possessive is rare in practice). Every other pair has different spelling: my/mine, your/yours, her/hers, our/ours, their/theirs. Learning these pairs as a set — not just memorizing the pronouns alone — helps you use the correct form automatically. When you're in doubt, apply the noun-removal test: if you can drop the noun and the sentence still works, use the pronoun form.