Questions & Answers about У мене болить потилиця.
Because the sentence is built differently from English.
In Ukrainian, with body-part pain, the painful body part is usually the grammatical subject, and the person is expressed separately with у/в + genitive:
- У мене болить потилиця.
- literally: At me the back of the head hurts
So Ukrainian is not saying I hurt. It is saying the back of my head hurts, with у мене showing whose body part it is.
Here мене is genitive singular after у.
The pattern у мене means something like at me / with me / in my case, and in this use у takes the genitive.
A useful thing to remember is that мене has the same form in both the genitive and accusative, but in this sentence it is functioning as genitive.