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.

It agrees with потилиця, not with I.

The subject of the sentence is потилиця = the back of the head, and that noun is singular, so the verb is singular too:

• потилиця болить = the back of the head hurts

The verb form болить is the present-tense singular form of боліти.

If the subject were plural, you would use болять:

• У мене болять плечі. = My shoulders hurt.

Потилиця is in the nominative singular.

It is the subject of the sentence, so nominative is exactly what you would expect here. The verb болить agrees with it in number.

Потилиця means the back of the head, roughly the nape/back-of-the-head area.

It is more specific than голова (head). So:

• голова = head in general
• потилиця = the back of the head

So this sentence is more specific than just У мене болить голова.

Yes. У and в are often just sound-based variants of the same preposition.

So both are possible:

• У мене болить потилиця.
• В мене болить потилиця.

The meaning does not change. Speakers usually choose whichever sounds smoother in context.

Yes, if the context already makes it clear that you are talking about yourself.

For example:

• Болить потилиця. = My back of the head hurts / The back of my head hurts.

This is very natural in conversation, especially if it is obvious who is speaking about their pain.

Adding у мене just makes the possessor explicit.

You can say it, and it is grammatical, but it is usually less neutral.

For body parts, Ukrainian often prefers the у мене pattern rather than a possessive adjective like моя. That is because body parts are an example of inalienable possession — things that are naturally part of you.

So the most natural everyday version is:

• У мене болить потилиця.

Моя потилиця болить sounds more emphatic or contrastive, as if you are stressing my back of the head in particular.

No, Ukrainian word order is fairly flexible.

The neutral, standard order here is:

• У мене болить потилиця.

But other orders are possible for emphasis:

• Потилиця у мене болить. → emphasizes потилиця
• Болить у мене потилиця. → more conversational/marked

So word order can change, but the neutral version for learners to use first is У мене болить потилиця.

Because Ukrainian has no articles.

There is no separate word for a/an or the. Whether something is definite or indefinite is understood from context.

So потилиця can be understood as the back of the head here without any article.

Then the verb changes to match a plural subject.

Compare:

• У мене болить потилиця. = singular
• У мене болять зуби. = plural
• У мене болять плечі. = plural

So the main pattern is:

• singular noun → болить
• plural noun → болять

It can cover both ideas, depending on context.

Боліти / болить is a general verb for pain:

• hurt
• ache
• sometimes be painful

If you want to make it stronger, you can add an adverb:

• дуже болить = hurts a lot
• сильно болить = hurts badly