Якщо в тебе болить коліно, краще не бігати і дати нозі відпочити.

Якщо means if and introduces a condition:

• Якщо в тебе болить коліно = If your knee hurts
• ..., краще не бігати... = ..., it’s better not to run...

So the whole sentence has the structure If X, then it’s better to do Y.

In Ukrainian, pain is very often expressed with the verb боліти = to hurt / to ache.

A very common pattern is:

• у/в + someone (genitive)
  • болить / болять
    • body part

So:

• в тебе болить коліно literally means something like at you, the knee hurts
• natural English: your knee hurts

This is much more natural in Ukrainian than building it around мати біль (to have pain).

Тебе here is in the genitive case after у / в in this possession-like structure.

Compare:

• ти = you
• тебе = of you / at you / you (genitive or accusative form depending on context)

In expressions like:

• у мене болить голова = my head hurts
• у тебе болить коліно = your knee hurts

the person is shown with у/в + genitive.

Yes. У тебе and в тебе are both correct here.

Ukrainian often alternates у and в for smoother pronunciation. The choice usually depends on sound and rhythm, not meaning.

So both are natural:

• Якщо у тебе болить коліно...
• Якщо в тебе болить коліно...

Because the subject is singular: коліно = knee.

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

So the verb agrees with коліно, not with тебе.

Because коліно is the grammatical subject of болить, so it is in the nominative case.

The idea is:

• What hurts? → коліно
• therefore nominative: коліно

That is why you get:

• в тебе болить коліно

not a different case form of коліно.

Краще means better and is used impersonally here, in the sense of it would be better.

So:

• краще не бігати = it’s better not to run

Very often Ukrainian leaves out words that English would express more explicitly. If you want, you can make the person more explicit:

• тобі краще не бігати = you’d better not run / it’s better for you not to run

In your sentence, тобі is simply understood from the context.

This is about aspect and the meaning of the infinitive.

• бігати = imperfective, more general/repeated activity: to run, to go running
• бігти = more like to be running / to run in one direction right now

In advice like this, Ukrainian usually uses the imperfective бігати because it means not to engage in running as an activity.

So:

• краще не бігати = it’s better not to run / not to go running

That sounds more natural than не бігти in this context.

Literally, it means to give the leg a chance to rest or to let the leg rest.

This is a very natural Ukrainian construction:

• дати комусь/чомусь + infinitive

Here:

• дати = to give / allow
• нозі = to the leg (dative case)
• відпочити = to rest

So:

• дати нозі відпочити = let the leg rest

English would not usually say give the leg to rest, but Ukrainian does use this pattern naturally.

Because нога is in the dative case after дати in this construction.

Forms of нога:

• nominative: нога
• accusative: ногу
• dative: нозі

Since the pattern is дати кому? чому? (give to whom? to what?), Ukrainian uses the dative:

• дати нозі відпочити

not дати ногу відпочити.

Because Ukrainian, like English, can move from a specific body part to a more general one.

• болить коліно = the pain is specifically in the knee
• дати нозі відпочити = let the whole leg rest

That sounds natural. It suggests that if the knee hurts, you should avoid strain on the entire leg.

You could imagine other phrasings, but this one is idiomatic and normal.

Відпочити means to rest and is the perfective infinitive.

Here the perfective makes sense because it refers to allowing the leg to have a period of rest, as a complete helpful action.

Compare:

• відпочивати = to be resting / to rest in an ongoing sense
• відпочити = to get some rest / to rest (successfully, as a whole action)

So дати нозі відпочити feels like let the leg get some rest.

Because the sentence begins with a subordinate if-clause:

• Якщо в тебе болить коліно, ...

In Ukrainian, clauses introduced by words like якщо are normally separated by a comma from the main clause.

So the structure is:

• Якщо ... , ...

just like English often writes:

• If your knee hurts, ...

It is mainly giving advice, not a direct command.

• краще не бігати = it’s better not to run
• дати нозі відпочити = let the leg rest / give the leg some rest

So the tone is gentle and practical, more like you should probably... than a strict order.

Yes. A fuller version could be:

• Якщо в тебе болить коліно, тобі краще не бігати й дати нозі відпочити.

This adds тобі (for you), which makes the advice more explicit. But the original sentence is already completely natural.

No. І means and, and it is perfectly natural here:

• не бігати і дати нозі відпочити

You may also see й instead of і, again mostly for smoother pronunciation:

• не бігати й дати нозі відпочити

The meaning is the same.