Moja partnerka woli spacerować po lesie niż oglądać telewizję.

Because moja has to agree with partnerka, which is a feminine singular noun.

In Polish, possessive words like my change form:

• mój = masculine
• moja = feminine
• moje = neuter

Since partnerka is feminine, the correct form is moja partnerka.

Partnerka is the feminine form of partner.

Depending on context, it can mean:

• a female romantic partner
• a girlfriend
• a female partner in some activity, business, dance, sport, etc.

In this sentence, most people would understand it as a romantic partner.

Woli is the 3rd person singular form of woleć, which means to prefer.

The subject is moja partnerka, so the verb must match she:

• ja wolę = I prefer
• ty wolisz = you prefer
• on/ona woli = he/she prefers

So Moja partnerka woli... means My partner prefers...

After woleć, Polish normally uses the infinitive when you are talking about preferring one action over another.

So:

• woli spacerować = prefers to walk
• niż oglądać telewizję = rather than watch TV

This is very similar to English:

• She prefers to walk rather than watch TV.

They are imperfective because the sentence expresses a general preference, not one single completed event.

• spacerować = to walk, to go walking
• oglądać = to watch

Imperfective verbs are the natural choice for habits, repeated actions, and general statements.

A perfective verb would suggest something more specific or completed, for example watching something all the way through once.

With spacerować, Polish very often uses po to express movement around an area.

So:

• spacerować po lesie = to walk around in the forest
• spacerować w lesie = to walk in the forest

Both are understandable, but po lesie sounds more natural when the idea is moving around through the forest.

Lesie is the locative singular form of las.

The preposition po takes the locative here, so:

• las → po lesie

This is why the noun changes form. The stem also changes a little:

• las
• w lesie
• po lesie

That kind of stem change is normal in Polish.

Niż means than in comparisons.

After woleć, it is commonly used to compare two options:

• woleć X niż Y = to prefer X rather than Y

Here the two things being compared are two actions:

• spacerować
• oglądać telewizję

So niż is the natural linking word.

Because oglądać takes a direct object, and direct objects usually go in the accusative case.

The noun is:

• telewizja = TV, television

Its accusative singular form is:

• telewizję

So:

• oglądać telewizję = to watch TV

In normal everyday Polish, oglądać telewizję means to watch TV.

If you wanted to talk about the physical device, you would usually say telewizor:

• oglądać telewizor would suggest looking at the TV set itself

So in this sentence, telewizję clearly means the activity of watching television.

Yes. Polish word order is more flexible than English word order.

The sentence as given is neutral and natural:

• Moja partnerka woli spacerować po lesie niż oglądać telewizję.

You can move parts around for emphasis, but the basic version is the best one for learners to copy first.

Yes, it sounds natural.

A Polish speaker would understand it immediately as:

• My partner prefers walking in the forest to watching TV.

The grammar, verb choice, and comparison structure are all normal and idiomatic.