Moja partnerka pracuje w biurze, a wieczorem lubi odpoczywać na balkonie.

Because moja has to agree with partnerka in gender, number, and case.

• partnerka is feminine singular
• so the possessive adjective must also be feminine singular: moja
• compare:
  • mój partner = my male partner
  • moja partnerka = my female partner

Here, partnerka is the subject, so it is in the nominative case.

Partnerka is the feminine form of partner.

In everyday modern Polish, partnerka often means:

• female partner
• girlfriend
• female romantic partner

Depending on context, it can also mean a female partner in other senses, such as in dancing or business, but in a sentence like this, most people would understand it as a romantic partner.

Polish often omits subject pronouns when they are not needed.

In English, you usually need she works.
In Polish, the verb form already gives information about the subject:

• pracuje = he/she/it works

And here the subject is explicitly named anyway:

• Moja partnerka pracuje...

So adding ona would usually be unnecessary unless you want extra emphasis:

• Ona pracuje w biurze... = She works in an office...

Pracuje is the 3rd person singular present tense form of pracować (to work).

So:

• ja pracuję = I work
• ty pracujesz = you work
• on/ona pracuje = he/she works

In this sentence, it describes a general or habitual fact, just like the English present simple:

• she works in an office
• in the evening she likes to relax on the balcony

This is a very common question, because Polish uses different prepositions depending on the place.

• w usually means in
• na usually means on or sometimes at

So:

• w biurze = in the office
• na balkonie = on the balcony

This matches English fairly well here. But in Polish, some place expressions are just learned as standard combinations. For example:

• w domu = at home / in the house
• na poczcie = at the post office
• na uniwersytecie = at the university

So part of this is meaning, and part of it is just usage you learn over time.

Because after w and na when talking about location, Polish normally uses the locative case.

Base forms:

• biuro = office
• balkon = balcony

Locative forms:

• w biurze = in the office
• na balkonie = on the balcony

So the noun changes form after the preposition.

This is very normal in Polish. For example:

• dom → w domu
• szkoła → w szkole
• park → w parku

In this sentence, a links two related pieces of information:

• Moja partnerka pracuje w biurze, a wieczorem lubi odpoczywać na balkonie.

Here, a is something like:

• and
• while
• sometimes a very light but

It often connects two facts with a slight contrast or shift of topic:

• she works in an office, and/while in the evening she likes to relax on the balcony

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

• pracuje w biurze i wieczorem lubi odpoczywać...

That is possible, but a sounds more natural when moving from one part of the day or one situation to another.

Wieczorem is a standard Polish way to say in the evening.

It is an adverbial time expression based on the instrumental form. You should mostly learn it as a fixed, very common pattern.

Examples:

• rano = in the morning
• wieczorem = in the evening
• nocą = at night / during the night

So:

• wieczorem lubi odpoczywać = she likes to relax in the evening

Depending on context, wieczorem can also feel like in the evenings when talking about a habit.

After lubić (to like), Polish normally uses the infinitive of another verb.

So:

• lubi odpoczywać = likes to relax / likes relaxing

Here:

• lubi = she likes
• odpoczywać = to rest / to relax

This is similar to English patterns like:

• likes to rest
• likes resting

You do not say lubi odpoczywa because the second verb is not separately conjugated here.

This is about aspect, which is very important in Polish.

• odpoczywać = imperfective
• odpocząć = perfective

In this sentence, the speaker is talking about a habit or general preference, so Polish normally uses the imperfective:

• lubi odpoczywać = she likes relaxing / likes to relax

The perfective odpocząć is more about a completed single result: to have a rest, to rest up. That is usually not the best choice after lubić when speaking about general habits.

So for general likes and repeated actions, odpoczywać is the natural form.

Yes. Polish word order is more flexible than English word order, because endings carry a lot of the grammatical information.

The sentence as given is very neutral and natural:

• Moja partnerka pracuje w biurze, a wieczorem lubi odpoczywać na balkonie.

But you could move things around for emphasis, for example:

• Wieczorem moja partnerka lubi odpoczywać na balkonie.
• Na balkonie moja partnerka lubi odpoczywać wieczorem.

These all mean roughly the same thing, but the focus changes. The original version is probably the best one for a beginner to learn first.

A rough English-friendly guide:

• moja ≈ MO-ya
• partnerka ≈ part-NER-ka
• pracuje ≈ pra-TSU-ye
• biurze ≈ BYOO-zhe
• wieczorem ≈ vye-CHO-rem
• lubi ≈ LOO-bee
• odpoczywać ≈ ot-po-CHI-vatch
• balkonie ≈ bal-KO-nye

A few useful pronunciation points:

• cz sounds like ch in chop
• rz usually sounds like zh in measure
• ć/ci sounds like a softer ch
• w in Polish sounds like English v

So wieczorem does not start with an English w sound.