Ci, którzy mieszkają na poddaszu, wiedzą, jak przebiega ewakuacja w ich budynku.

Ci, którzy means those who.

• ci = those
• którzy = who

Polish often uses this two-part structure where English uses just one phrase. So:

• Ci, którzy mieszkają na poddaszu... = Those who live in the attic/top floor...

Also, ci and którzy are in the masculine personal plural form, so they are used for a group of people that includes at least one man, or a mixed group. If the group were entirely female, you would normally say:

• Te, które mieszkają na poddaszu...

Because którzy is the standard relative pronoun for people in careful, standard Polish.

So:

• ci, którzy mieszkają... = standard / formal / neutral
• ci, co mieszkają... = common in speech, but more colloquial

A learner should usually treat którzy as the safer and more standard choice.

Because the subject is plural: ci, którzy... = those who...

So both verbs agree with that plural subject:

• mieszkają = they live
• wiedzą = they know

If the sentence were about one person, you would get singular forms:

• Ten, kto mieszka na poddaszu, wie... = The one who lives in the attic/top floor knows...

Because this sentence describes location, not movement.

With na:

• na + locative = being somewhere
• na + accusative = going onto/to somewhere

Here the people live there, so we use the locative:

• na poddaszu = in/on the attic space / top floor

Compare:

• Mieszkają na poddaszu. = They live in the attic/top floor.
• Weszli na poddasze. = They went up to the attic.

The basic form is poddasze, and poddaszu is its locative singular form.

Not always exactly.

Poddasze is the space directly under the roof. In practice, it can mean:

• an attic
• a loft
• a top-floor space under the roof
• sometimes a finished attic apartment

So in this sentence, it likely means the people who live in the uppermost part of the building, under the roof.

A related word is strych, which more strongly suggests a traditional storage attic.
So:

• poddasze = attic space / loft area, often usable or lived in
• strych = attic, often more like storage space

Here wiedzą is the natural choice because it means they know a fact / they know how something works.

• wiedzieć = to know information
• znać = to know a person, place, thing, or be familiar with something

So:

• wiedzą, jak przebiega ewakuacja = they know how the evacuation proceeds
• znają ewakuację would not sound natural in the same way

You could say something like:

• Znają procedurę ewakuacji. = They know the evacuation procedure.

But with jak... Polish normally uses wiedzieć.

Very literally, przebiegać means something like to run, to proceed, or to unfold.

So:

• jak przebiega ewakuacja = how the evacuation proceeds
• more natural English: how the evacuation works or what the evacuation process is like

This is a very common use of przebiegać with processes, events, lessons, meetings, exams, etc.

For example:

• Jak przebiega spotkanie? = How does the meeting go?
• Jak przebiega leczenie? = How is the treatment progressing?

Because ewakuacja is the subject of the subordinate clause:

• jak przebiega ewakuacja w ich budynku

In that clause:

• ewakuacja = the thing that proceeds
• przebiega = proceeds / takes place

So ewakuacja stays in the nominative singular.

Even though the whole clause depends on wiedzą (they know), inside that clause ewakuacja still functions as its own subject.

Because przebiega agrees with ewakuacja, not with the people.

In the subordinate clause, the grammatical subject is:

• ewakuacja = singular

So the verb is also singular:

• ewakuacja przebiega = the evacuation proceeds

The people are the subject of the main clause:

• Ci... wiedzą... = Those people know...

So the sentence has two different subject-verb relationships:

1. Ci ... wiedzą
2. ewakuacja przebiega

Because the pronoun is inside a clause where the grammatical subject is ewakuacja, not those people.

In Polish, swój usually refers to the subject of its own clause. But in:

• jak przebiega ewakuacja w ich budynku

the subject of that clause is:

• ewakuacja

So swój budynek would suggest, grammatically, that the building somehow belongs to the evacuation, which does not make sense.

That is why ich budynku is used: it clearly means their building, referring back to the people in the main clause.

Because Polish punctuation normally separates subordinate clauses with commas.

There are two subordinate parts here:

1. którzy mieszkają na poddaszu
   This is a relative clause describing ci.

2. jak przebiega ewakuacja w ich budynku
   This is an indirect question depending on wiedzą.

So the commas help mark the clause boundaries:

• Ci, którzy mieszkają na poddaszu, wiedzą, jak przebiega ewakuacja w ich budynku.

Polish generally uses commas in such places more consistently than English does.

Yes, Polish word order is fairly flexible, though the original version is very natural and neutral.

For example, you could also say:

• Ci, którzy mieszkają na poddaszu, wiedzą, jak ewakuacja przebiega w ich budynku.
• Ci, którzy mieszkają na poddaszu, wiedzą, jak w ich budynku przebiega ewakuacja.

These versions are possible, but they may sound slightly more marked or less neutral. The original:

• Ci, którzy mieszkają na poddaszu, wiedzą, jak przebiega ewakuacja w ich budynku.

is a very standard way to say it.

It sounds like a general fact.

The present tense here suggests:

• these people know the evacuation procedure
• they know how evacuation works in their building in general

If you wanted to talk about one specific evacuation event, Polish would normally add more context, for example:

• wiedzą, jak przebiegała ewakuacja = they know how the evacuation went
• wiedzą, jak będzie przebiegać ewakuacja = they know how the evacuation will proceed

So the original sentence sounds like a general statement about preparedness or building safety.