A Friend of Mine
Use the double possessive construction — indefinite article + noun + of + possessive pronoun — to refer to one of several people or things belonging to someone.
The Double Possessive: 'a friend of mine'
The double possessive (also called the post-genitive or 'of mine' construction) is a distinctive feature of English that learners often avoid or misuse. Learner corpus data shows that errors involving 'of + possessive' are most common at B1 level, with two recurring patterns: using the object pronoun ('a friend of me') or using the possessive adjective ('a friend of my') instead of the required possessive pronoun.
The Structure
Indefinite article + noun + of + possessive pronoun (mine / yours / his / hers / ours / theirs)
Is that a student of yours? (= one of your students)
Some friends of ours are coming tonight. (= some of our friends)
That brilliant idea of his saved the project. (= one of his ideas)
Why 'a my friend' Is Wrong
English does not allow a possessive adjective directly after an indefinite article. 'A my friend' violates the rule that adjectives cannot stack with possessives before a noun. The 'of' construction is the grammatical workaround: it allows indefiniteness (a/an/some/any) and possession to coexist.
Pronoun, Not Adjective, After 'of'
After 'of', always use the pronoun form. Compare: 'A photo of her' (= a photograph depicting her) vs 'A photo of hers' (= a photograph belonging to her). The difference is ownership vs depiction.
Common Mistakes
✗ Is it a photo of your? → ✓ Is it a photo of yours?
✗ I saw a friend of her. → ✓ I saw a friend of hers.