Nasza sąsiadka zna dobrego fachowca i mówi, że remont potrwa tylko trzy dni.

Because sąsiadka is a feminine noun, meaning female neighbor / neighbor woman. The possessive adjective nasz (our) has to agree with the noun in gender, number, and case.

So:

• nasz sąsiad = our male neighbor
• nasza sąsiadka = our female neighbor

Here, both words are in the nominative singular:

• nasza = feminine nominative singular
• sąsiadka = feminine nominative singular

It is in the nominative case because it is the subject of the sentence.

The person doing the actions is our neighbor, and she:

• zna = knows
• mówi = says

So nasza sąsiadka is the subject, which is why nominative is used.

This is a very common question. In this sentence, dobrego fachowca is actually accusative, not genitive.

The verb znać (to know / be acquainted with) takes the accusative case.

So:

• zna kogo? co? = knows whom? what?

The noun fachowiec (specialist, skilled worker, professional) is a masculine animate noun. For masculine animate nouns in the singular, the accusative form is the same as the genitive form.

That is why you get:

• nominative: dobry fachowiec
• accusative: dobrego fachowca
• genitive: dobrego fachowca

So it looks like genitive, but here it is functioning as accusative because of znać.

Fachowiec means something like:

• skilled worker
• professional
• specialist
• expert tradesperson

In the context of remont (renovation / repair work), it usually means a reliable person who knows how to do building, repair, or renovation work well.

So dobry fachowiec is often a very practical phrase in Polish for a good contractor / skilled repair person.

In Polish, a clause introduced by że (that) is normally separated by a comma.

So:

• mówi, że... = says that...

This is standard Polish punctuation. English sometimes drops that, but Polish usually keeps że in this kind of sentence, and the comma is expected.

Because Polish generally uses że to introduce a content clause after verbs like:

• mówić = to say
• myśleć = to think
• wiedzieć = to know
• uważać = to believe / think

So:

• Mówi, że remont potrwa trzy dni. = She says that the renovation will take three days.

In everyday speech, Polish is usually less likely than English to omit that in this kind of sentence.

Because potrwać is a perfective verb, and perfective verbs form the future with a simple future form.

Here is the pair:

• trwać = to last, to be ongoing (imperfective)
• potrwać = to last for some time / to take some time and come to an end (perfective)

So:

• remont trwa = the renovation is going on / is lasting
• remont będzie trwał = the renovation will be lasting / will last
• remont potrwa trzy dni = the renovation will take three days / will last three days

In this sentence, potrwa is very natural because the speaker is talking about the total duration up to completion.

In this verb, po- helps create a perfective meaning. It suggests a bounded action: something lasts for a certain period and then is finished.

So:

• trwać focuses on the ongoing duration
• potrwać focuses on the fact that it will last for some amount of time and then end

That is why remont potrwa trzy dni sounds like:

• the renovation will take three days
• the renovation will last three days

Because duration expressions after verbs like trwać / potrwać often use the accusative.

So:

• dzień = day
• trzy dni = three days

In this sentence, tylko trzy dni expresses how long the renovation will last.

Compare:

• Remont potrwa trzy dni. = The renovation will take three days.
• Nie było go przez trzy dni. = He wasn’t there for three days.

Polish often uses the accusative for a span of time.

The word order here is very natural and neutral:

Nasza sąsiadka zna dobrego fachowca i mówi, że remont potrwa tylko trzy dni.

Polish word order is fairly flexible because case endings show grammatical roles, but changing the order changes emphasis.

For example:

• Nasza sąsiadka zna dobrego fachowca...
  Neutral: Our neighbor knows a good professional...

• Dobrego fachowca zna nasza sąsiadka...
  More emphasis on dobrego fachowca

• Tylko trzy dni potrwa remont.
  More emphasis on only three days

So yes, it can be changed, but the original version is the most straightforward.

Not always. In Polish, znać often means:

• to know a person
• to be acquainted with
• to know something from experience
• to know how something is / be familiar with

Here, zna dobrego fachowca means:

• she knows a good contractor
• she is acquainted with a good skilled worker

You would not normally use znać for factual knowledge in the same way as English know that...
For example:

• Znam tego człowieka. = I know that man.
• Wiem, że on przyjdzie. = I know that he will come.

So:

• znać kogoś/coś = know someone / be familiar with something
• wiedzieć, że... = know that...

Yes. Remont can refer to:

• renovation
• repair work
• refurbishment
• remodeling work

The exact translation depends on context. In everyday Polish, remont often means any significant work done in a flat, house, or building, such as painting, replacing tiles, redoing a bathroom, and so on.

So remont potrwa tylko trzy dni could mean:

• the renovation will take only three days
• the repair work will last only three days

Because Polish often expresses the future through the verb form itself.

Here, potrwa is already a future tense form of a perfective verb. So it already means will last / will take.

English needs a separate helper word:

• will last

Polish does not here:

• potrwa

This is one of the important differences between English and Polish tense formation.

Yes, it is in a very natural place. Tylko means only, and here it modifies trzy dni:

• potrwa tylko trzy dni = it will last only three days

That means the duration is short or shorter than expected.

You could move tylko for emphasis in some contexts, but the given placement is the most natural neutral one.

A few pronunciation points that English speakers often notice:

• nasza: sz sounds like English sh
• sąsiadka: ą is a nasal vowel; before some consonants it may sound a bit like on/om, though not exactly
• zna: the z is pronounced; this is not like English silent letters
• dobrego: stress is on the second-to-last syllable: do-BRE-go
• fachowca: ch sounds like Polish h/ch, not like English ch in chair
• mówi: ó sounds like u
• że: ż sounds like the s in measure
• remont: stress on the second-to-last syllable: re-MONT
• potrwa: the cluster trw can feel difficult at first
• trzy: trz is a tough consonant cluster for English speakers

Also, Polish usually stresses the penultimate syllable:

• na-SZA
• są-SIAD-ka
• fa-CHOW-ca
• po-TRWA
• TYL-ko
• DNI is one syllable

That is mostly context, not grammar. Grammatically, the sentence simply says:

• our neighbor knows a good professional
• and says the renovation will take only three days

But pragmatically, many listeners may infer that:

• the neighbor is being helpful,
• she has a useful contact,
• and she is passing on reassuring information.

So the extra meaning comes from the situation, not from any special grammatical form.