Mój kuzyn jest piłkarzem, a moja kuzynka jest piłkarką w tej samej szkole.

Because after jest in this kind of sentence, Polish usually uses the instrumental case for professions, roles, and identities.

So:

• piłkarz = nominative form
• piłkarzem = instrumental form
• piłkarka = nominative form
• piłkarką = instrumental form

That is why you get:

• Mój kuzyn jest piłkarzem.
• Moja kuzynka jest piłkarką.

This is very common in Polish:

• On jest nauczycielem. = He is a teacher.
• Ona jest studentką. = She is a student.

Because mój / moja / moje must agree with the noun in gender, number, and case.

Here:

• kuzyn is masculine, so you use mój
• kuzynka is feminine, so you use moja

So:

• mój kuzyn = my male cousin
• moja kuzynka = my female cousin

This kind of agreement happens all the time in Polish:

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

They mean cousin, but Polish usually distinguishes gender:

• kuzyn = male cousin
• kuzynka = female cousin

English often just says cousin, but Polish normally makes the gender clear.

Because the preposition w meaning in or at usually requires the locative case when it refers to location.

The basic forms are:

• ta szkoła = this school
• ta sama szkoła = the same school

But after w, they change to locative:

• w tej szkole = in/at this school
• w tej samej szkole = in/at the same school

So all the words in that phrase have to match the feminine singular locative form:

• ta → tej
• sama → samej
• szkoła → szkole

tej samej means the same here, but it is the locative feminine singular form of ten sam.

The base idea is:

• ten sam = the same

But Polish changes the form depending on gender and case. Since szkoła is feminine and comes after w, you get:

• w tej samej szkole = in/at the same school

So yes, it means the same, but in a form that agrees with szkole.

Both can often translate as and, but they are not used in exactly the same way.

In this sentence, a is natural because it links two parallel statements with a slight contrast or comparison:

• Mój kuzyn jest piłkarzem, a moja kuzynka jest piłkarką...

It feels a bit like:

• My male cousin is a football player, and my female cousin is a football player...
• or ..., while my female cousin...

By contrast:

• i is more like simple and
• a often feels like and / while / whereas
• ale means but

So a is a very natural choice here.

Usually, in Polish, piłkarz and piłkarka normally mean a soccer player / association football player.

So for most learners, the safest understanding is:

• piłkarz = male soccer player
• piłkarka = female soccer player

If you want to talk about American football, Polish normally says something more specific, such as futbol amerykański.

Yes, you can, but the structure is different.

Two common patterns are:

• Mój kuzyn jest piłkarzem.
• Mój kuzyn to piłkarz.

Both can mean My cousin is a football player, but:

• jest + instrumental is the standard pattern for saying what someone is
• to + nominative is also common, especially for identification or definitions

So:

• jest piłkarzem → uses instrumental
• to piłkarz → uses nominative

For learners, jest + instrumental is a very important pattern to know.

No, Polish word order is fairly flexible, but the given sentence is the most neutral and natural order.

The original sentence:

• Mój kuzyn jest piłkarzem, a moja kuzynka jest piłkarką w tej samej szkole.

This sounds normal and clear.

You could move some parts for emphasis, for example:

• W tej samej szkole moja kuzynka jest piłkarką.

But that changes the focus slightly. Polish often uses word order to highlight information rather than to mark basic grammar.

So the original version is probably the best default model for a learner.

Because Polish does not have articles like English.

So Polish simply says:

• mój kuzyn = my cousin
• jest piłkarzem = is a football player
• w tej samej szkole = in the same school

Whether something means a, the, or just a general idea is understood from context.

That is completely normal in Polish, even though it can feel strange to English speakers at first.

A few sounds here are especially important:

• ł is pronounced like English w
  • so piłkarz sounds roughly like peew-kazh
• ą is a nasal vowel
  • at the end of piłkarką, it sounds roughly like -kon or a nasal o, depending on pronunciation style
• rz usually sounds like the zh sound in measure
  • so piłkarz ends roughly like kazh

Very rough pronunciation guide:

• Mój kuzyn ≈ mooy KOO-zin
• jest piłkarzem ≈ yest pew-KA-zhem
• moja kuzynka ≈ MO-ya koo-ZIN-ka
• jest piłkarką ≈ yest pew-KAR-kon
• w tej samej szkole ≈ f tey SA-mey SHKO-le

These are only approximations, but they help with the most unfamiliar sounds.