Questions & Answers about У мене болить зап’ястя.
This is a very common Ukrainian pattern.
With pain, sensations, and many physical states, Ukrainian often says:
У/В + person + болить/болять + body part
So У мене болить зап’ястя is literally something like At me, the wrist hurts, but naturally it means My wrist hurts.
Using я here would not sound natural.
Мене is the genitive form of я.
The preposition у (or в) requires that form in this construction:
- я = I
- мене = me / of me / at me, depending on the structure
So in У мене болить зап’ястя, мене is there because Ukrainian expresses this idea with у + genitive.