A possessive cannot be put before another determiner and a noun . One can say John’s friend, my friend, a friend or that friend but one cannot say a John’s friend, that my friend, the construction of + possessive is used.
Examples:
A friend of mine is coming to stay next week.
That neighbour of his saved his life.
It was a good idea of yours to go to the clinic.
On can also say a friend of John’s or a friend of my sister’s.
Examples:
The person who has bought the house next door to you, is a psychiatrist.
It was a good idea of Jim’s to take her temperature .
Own
Own can only be used after a possessive word – it cannot follow an article ie: an own house.
Examples:
Fred has just bought his own car.
It’s my own fault that it’s so cold in here, I forgot to shut the window.
Unfortunately the flat hasn’t got it’s own garage, so we have to park on the street.
Own can also be used to say that you do something yourself, rather than someone else doing it for you.