Po treningu boli mnie ucho, bo wiatr był bardzo zimny.

Because po in the sense of after normally takes the locative case in Polish.

So:

• trening = training / workout
• po treningu = after training / after the workout

This is a very common pattern:

• po pracy = after work
• po obiedzie = after lunch
• po lekcji = after class

So treningu is the locative singular form of trening.

Polish often expresses pain with the verb boleć in a structure that is closer to the ear hurts me.

In this sentence:

• ucho is the thing that hurts
• mnie is the person affected

So literally, the structure is roughly:

• ucho boli mnie = the ear hurts me

But the most natural English translation is still my ear hurts.

This is a very common Polish pattern:

• Boli mnie głowa = My head hurts
• Boli mnie ręka = My arm hurts
• Bolą mnie zęby = My teeth hurt

Because boleć normally takes the affected person in the accusative, not the dative.

So:

• mnie = accusative/genitive form of ja
• mi = dative form

With boleć, standard Polish uses:

• Boli mnie ucho
• Bolą mnie plecy
• Boli cię głowa

not mi in this structure.

A useful way to remember it is that boleć behaves like something is acting on you: it hurts me.

Because in Polish, with body parts, it is very common to leave out the possessive adjective if the owner is already clear.

So Polish prefers:

• Boli mnie ucho

rather than:

• Boli mnie moje ucho

The word mnie already shows whose ear it is, so moje is usually unnecessary.

Adding moje would sound emphatic or contrastive, for example if you were comparing your ear with someone else’s.

Because ucho is the subject of boli, so it stays in the nominative.

In boli mnie ucho:

• ucho = nominative singular
• mnie = accusative

The verb agrees with ucho, which is why we get boli in the singular.

If the painful thing were plural, the verb would change too:

• Bolą mnie uszy = My ears hurt

Notice:

• singular: boli
• plural: bolą

Also, ucho is an irregular noun in the plural:

• singular: ucho
• plural: uszy

Because here the speaker is talking specifically about the wind:

• wiatr = wind

Since wiatr is a masculine singular noun, the past tense verb and adjective agree with it:

• wiatr był bardzo zimny = the wind was very cold

Breakdown:

• był = was, masculine singular
• zimny = cold, masculine singular adjective

By contrast, było bardzo zimno means it was very cold in a general weather sense, with no specific noun like wiatr as the subject.

So the difference is:

• Wiatr był bardzo zimny = The wind was very cold
• Było bardzo zimno = It was very cold

Because the sentence refers to a past situation: the wind was very cold earlier, during or around the training.

The verb być in the past tense changes for gender and number:

• był = masculine singular
• była = feminine singular
• było = neuter singular
• byli / były = plural

Since wiatr is masculine singular, Polish uses był.

Because bo introduces a reason clause, and in standard Polish punctuation, it is preceded by a comma.

So:

• Po treningu boli mnie ucho, bo wiatr był bardzo zimny.

This is similar to English because, although Polish comma rules are stricter here.

Also, bo is a very common everyday word meaning because. A more formal alternative would be ponieważ.

Yes. Polish word order is fairly flexible, because cases show grammatical relationships.

The original sentence:

• Po treningu boli mnie ucho, bo wiatr był bardzo zimny.

is natural because po treningu sets the time context first.

Other orders are possible, for example:

• Boli mnie ucho po treningu, bo wiatr był bardzo zimny.
• Ucho mnie boli po treningu, bo wiatr był bardzo zimny.

But they may sound more marked, depending on emphasis.

In general:

• sentence-initial Po treningu = nice, natural topic-setting phrase
• moving ucho forward can add emphasis to the ear itself

Because Polish does not have articles like English a/an and the.

So nouns appear without them:

• treningu
• ucho
• wiatr

Whether something is understood as a or the depends on context.

For example:

• wiatr był bardzo zimny could mean the wind was very cold
• in another context, it might be understood more generally as a cold wind

English speakers often look for articles in Polish, but they simply are not there.