Після довгої прогулянки в неї болить коліно.

Because Ukrainian uses a different pattern for someone has a pain / something hurts on someone.

The usual structure is:

у / в + person + болить / болять + body part

So:

• У мене болить голова = My head hurts.
• У нього болить спина = His back hurts.
• В неї болить коліно = Her knee hurts / She has knee pain.

Here, в неї does not mean “inside her.” It is part of a very common idiomatic pattern meaning roughly for her / in her case.

If you say вона болить коліно, that is not grammatical Ukrainian.

After the preposition в / у, the pronoun must change form.

The pronoun вона has these relevant forms:

• вона = she
• її = her
• неї = her, after certain prepositions

So after a preposition, Ukrainian uses:

• у неї
• до неї
• від неї
• на неї

This extra н- appears with 3rd-person pronouns after prepositions. It is a normal grammar rule.

So:

• вона = she
• її = her
• в неї = at her / for her / with her

Both в and у are often just pronunciation variants in Ukrainian. Speakers choose the one that sounds smoother.

So both of these are possible:

• В неї болить коліно
• У неї болить коліно

The choice often depends on surrounding sounds and rhythm. In many cases, either one is acceptable.

Because після always takes the genitive case.

The base form is:

• довга прогулянка = a long walk

After після, both words must go into the genitive singular:

• довгої прогулянки = after a long walk

So:

• після
  • genitive
• після уроку = after the lesson
• після роботи = after work
• після довгої прогулянки = after a long walk

Because прогулянка is a feminine singular noun, and after після it must be in the genitive singular.

Here is the change:

• довга прогулянка = nominative
• довгої прогулянки = genitive

So the endings change like this:

• довга → довгої
• прогулянка → прогулянки

This is normal adjective-noun agreement: the adjective must match the noun in gender, number, and case.

Because the grammatical subject is коліно, which is singular.

In Ukrainian, the body part is usually the subject of боліти:

• коліно болить = the knee hurts
• очі болять = the eyes hurt

So:

• singular subject → болить
• plural subject → болять

Examples:

• У мене болить рука = My arm hurts.
• У мене болять руки = My arms hurt.

Because коліно is the subject of the verb болить, not the object.

The structure is basically:

• коліно болить = the knee hurts

That is why коліно stays in the nominative case.

This is similar to English My knee hurts, where knee is also the subject.

Її коліно болить is grammatically possible, but it is a different structure and sounds less natural in many everyday situations.

Compare:

• В неї болить коліно = She has knee pain / Her knee hurts.
• Її коліно болить = Her knee hurts.

The first version is the standard everyday pattern for talking about pain in Ukrainian. It focuses on the person experiencing the pain.

The second version puts more focus on her knee itself. It is not wrong, but it is less idiomatic as the basic way to say someone has a pain.

Yes. Ukrainian word order is fairly flexible.

This sentence could also appear as:

• В неї після довгої прогулянки болить коліно.
• Коліно в неї болить після довгої прогулянки.
• Після довгої прогулянки у неї болить коліно.

The basic meaning stays the same, but the emphasis changes.

The original order sounds natural and neutral: Після довгої прогулянки в неї болить коліно.

It sets the time/context first: after a long walk.

Not exactly.

• болить is a verb: hurts / aches
• боляче is an adverb/predicative word: painful / it hurts

Compare:

• У мене болить коліно. = My knee hurts.
• Мені боляче. = I’m in pain / It hurts.

So in this sentence, болить is correct because the speaker is saying that a specific body part hurts.

Yes, if you mean both knees or knees in general.

Compare:

• болить коліно = one knee hurts
• болять коліна = the knees hurt

So the singular/plural depends on what is hurting:

• У неї болить коліно.
• У неї болять коліна.

Ukrainian has no articles, so the phrase itself does not directly mark a vs the.

So після довгої прогулянки can mean:

• after a long walk
• after the long walk

The exact interpretation comes from context. In many learning examples, English translates it as after a long walk.

The dictionary forms are:

• після = after
• довгий = long
• прогулянка = walk, stroll
• вона = she
• боліти = to hurt, to ache
• коліно = knee

The forms in the sentence are changed because of grammar:

• довгий → довгої
• прогулянка → прогулянки
• вона → неї
• боліти → болить
• коліно stays коліно