Mój brat ćwiczy na gitarze, a jego koleżanka gra na pianinie.

Because mój has to agree with the noun it describes.

• brat = brother
• brat is masculine singular
• so the correct form is mój = my for a masculine singular noun in the nominative

Compare:

• mój brat = my brother
• moja siostra = my sister
• moje dziecko = my child

So mój brat is correct because brat is masculine.

Ćwiczy means practises / is practising.

It comes from the verb ćwiczyć = to practise, to exercise, to train.

In this sentence:

• Mój brat ćwiczy na gitarze = My brother practises on the guitar / more naturally in English, My brother is practising guitar

A useful point: ćwiczyć focuses on practice, while grać often means to play an instrument more generally.

So there is a small difference between:

• ćwiczy na gitarze = he is practising guitar
• gra na gitarze = he plays the guitar / is playing the guitar

Because with many musical instruments, Polish uses the pattern:

• grać na + locative case
• ćwiczyć na + locative case

So:

• gitara → na gitarze
• pianino → na pianinie

This is a very common Polish structure meaning on the guitar, on the piano, but in natural English we usually just say play the guitar or play the piano.

Examples:

• Gram na gitarze. = I play the guitar.
• Ona gra na pianinie. = She plays the piano.
• On ćwiczy na skrzypcach. = He practises the violin.

So the important thing to remember is that na is normal here in Polish.

Because after na in this meaning, Polish uses the locative case.

That causes noun endings to change:

• gitara → na gitarze
• pianino → na pianinie

This is not random; it is a grammar pattern.

A beginner-friendly way to remember it is:

• dictionary form: gitara, pianino
• after na when talking about playing an instrument: na gitarze, na pianinie

You do not need to translate the case directly into English. Just learn the whole phrase:

• grać na gitarze
• grać na pianinie

In Polish, a often links two ideas by contrasting them lightly or by setting them side by side.

Here:

• Mój brat ćwiczy na gitarze, a jego koleżanka gra na pianinie.

This means something like:

• My brother practises guitar, while his female friend plays the piano.

If you used i, it would sound more like simple addition:

• My brother practises guitar and his friend plays the piano.

So:

• i = and
• a = and / while / whereas, depending on context

In this sentence, a is very natural because it compares two different people doing two different things.

Because in Polish, a usually introduces another clause, and it is normally preceded by a comma.

So this is standard punctuation:

• Mój brat ćwiczy na gitarze, a jego koleżanka gra na pianinie.

This is similar to English when we write:

• My brother practises guitar, and his friend plays the piano.
• My brother practises guitar, while his friend plays the piano.

So yes, the comma here is expected.

Jego koleżanka means his female friend, his female classmate, his female colleague, or his female acquaintance, depending on context.

Important vocabulary:

• kolega = a male friend / male classmate / male colleague
• koleżanka = a female friend / female classmate / female colleague

It does not automatically mean a romantic partner.

So jego koleżanka is not the same as:

• jego dziewczyna = his girlfriend

In many situations, koleżanka is best understood as female friend or female acquaintance.

Jego means his, and in this sentence it means his female friend/colleague.

A useful thing for learners is that jego is much less visibly changeable than mój in many common forms. In this sentence, you simply learn:

• jego brat = his brother
• jego siostra = his sister
• jego koleżanka = his female friend

So unlike mój / moja / moje, the form jego stays the same here.

That is why we say:

• mój brat
• jego koleżanka

Because the two verbs express slightly different ideas.

• ćwiczyć = to practise
• grać = to play

So the sentence is not perfectly parallel on purpose:

• Mój brat ćwiczy na gitarze = My brother is practising on the guitar
• jego koleżanka gra na pianinie = his female friend is playing the piano / plays the piano

The speaker may want to show that:

• the brother is doing practice
• the female friend is simply playing

Of course, you could also say:

• Mój brat gra na gitarze.
• Jego koleżanka ćwiczy na pianinie.

Both verbs work with instruments, but they do not mean exactly the same thing.

They are both in the present tense, third person singular.

That means:

• ćwiczę = I practise
• ćwiczysz = you practise
• ćwiczy = he/she practises

and

• gram = I play
• grasz = you play
• gra = he/she plays

So in this sentence:

• brat ćwiczy = brother practises / is practising
• koleżanka gra = female friend plays / is playing

Like in Polish generally, the same present-tense form can often be translated into English as either:

• practises / plays or
• is practising / is playing

depending on context.

No, Polish word order is fairly flexible, but this version is the most neutral and natural.

The sentence:

• Mój brat ćwiczy na gitarze, a jego koleżanka gra na pianinie.

has a straightforward pattern:

• subject + verb + phrase
• subject + verb + phrase

Because Polish uses case endings, word order can change more easily than in English. For example, you might also hear:

• Na gitarze ćwiczy mój brat, a na pianinie gra jego koleżanka.

That puts more emphasis on the instruments.

But for a learner, the original version is the best neutral model.

Yes, literally it does. But you should not translate it word for word in normal English.

Polish says:

• grać na gitarze
• grać na pianinie

Literally: to play on the guitar/piano

Natural English:

• to play the guitar
• to play the piano

So the literal meaning can help you understand the structure, but the natural translation in English is different.

A rough English-style guide:

• mój ≈ mooy
• brat ≈ braht
• ćwiczy ≈ CHVEE-chih
• gitarze ≈ gee-TA-rzheh
• jego ≈ YEH-go
• koleżanka ≈ ko-leh-ZHAN-ka
• gra ≈ grah
• pianinie ≈ pya-nee-NEE-nyeh

A few important sounds:

• ć is a soft ch sound
• ż sounds like the s in measure
• j is like English y
• ń is a soft ny sound

If you want to sound more natural, remember that Polish stress is usually on the second-to-last syllable:

• ko-le-ŻAN-ka
• pia-ni-NI-nie
• gi-TA-rze