I don't really understand why we say
I have a daughter
using the "a" article. It's not a random person in the street, it's not one of my 78 daughters, this is the only daughter I have. I have no other family members, for example. Why would I still have to say "a daughter"? Is there any secret rule?:)
As I understand "a" can mean two things:
1) One 2) Some-thing\-body unknown, indefinite, some
There can be two situations when I have a daughter.
1) Me and my daughter are the closest people for each other. I don't have any other children. I help her with her work, she visits me twice a week, I know all of her secrets, her boyfriend, by the way, he is the coolest guy I've ever met, her plans for the future and so on. I know my daughter as nobody else. So, yeah, I have a(=one) daughter.
2) The last time I saw my daughter 20 years ago. Now I have no idea what she does, shere she lives, whether she has any kids even whether she is alive. I've not been having any contact with her for all these years. So, yeah, I have a(=some, unknown, indefinite) daughter somewhere.
And I am stuck with which variant people will understand if I say "I have a daughter".