Na autostradzie trzeba uważać bardziej niż w mieście, zwłaszcza gdy prędkość rośnie.

Because Polish uses different prepositions with different kinds of places.

• na autostradzie = on the motorway / highway
• w mieście = in the city

This is mostly idiomatic and has to be learned as part of the noun phrase:

• na drodze = on the road
• na ulicy = on the street
• na autostradzie = on the motorway
• w mieście = in the city
• w domu = in the house / at home

So even though English says on for roads and in for cities too, the important thing is that Polish chooses na with autostrada and w with miasto.

They are both in the locative case, because they follow prepositions that require it here:

• na autostradzie
• w mieście

The dictionary forms are:

• autostrada → autostradzie
• miasto → mieście

So the pattern is:

• na + locative
• w + locative

This is why you do not see autostrada or miasto in their basic forms in this sentence.

Trzeba means something like:

• one must
• it is necessary
• you have to

It is an impersonal word. That means there is no clear subject like I, you, or we.

So:

• Trzeba uważać = You have to be careful / One must be careful / It is necessary to be careful

This is a very common Polish structure for giving general advice or stating necessity.

Because after trzeba, Polish normally uses the infinitive.

So:

• trzeba uważać = one must be careful
• trzeba pracować = one must work
• trzeba iść = one must go

This is similar to English have to + verb or must + verb, except Polish uses the impersonal trzeba plus the infinitive.

In this sentence, uważać means:

• to be careful
• to pay attention
• to watch out

So trzeba uważać is not just abstract to consider; it is practical, like staying alert and being cautious while driving.

In traffic-related contexts, uważać very often means be careful.

Bardziej means more, and niż means than.

So:

• bardziej niż w mieście = more than in the city

In the full sentence, it means you need to be careful more on the motorway than in the city.

A useful pattern is:

• bardziej ... niż ... = more ... than ...

For example:

• On pracuje bardziej niż ja = He works more than I do.
• Trzeba uważać bardziej niż wcześniej = You need to be more careful than before.

Because uważać is a verb, and Polish usually compares the degree of the action with an adverb like bardziej.

So:

• uważać bardziej = be careful more / be more careful

You do not make a comparative form out of the verb itself. Instead, you add bardziej.

This is similar to English using more:

• be more careful
• pay more attention

Zwłaszcza means especially.

It adds emphasis:

• Na autostradzie trzeba uważać bardziej niż w mieście = You need to be more careful on the motorway than in the city.
• ..., zwłaszcza gdy prędkość rośnie = ..., especially when speed increases.

So the sentence first gives a general statement, then adds a situation where that statement is even more true.

Both can mean when.

In this sentence:

• gdy prędkość rośnie = when speed increases

A few general notes:

• kiedy is very common in everyday speech
• gdy can sound a little more neutral, literary, or formal, but it is also common and natural

In many sentences, they are interchangeable:

• zwłaszcza gdy prędkość rośnie
• zwłaszcza kiedy prędkość rośnie

Both are correct.

Literally, it means:

• speed grows
• more naturally: speed increases

So:

• prędkość = speed
• rośnie = grows / rises / increases

In context, it means that as your speed becomes higher, you need to be even more careful.

Because the sentence is expressing a general truth or general rule.

Polish often uses the present tense for things that are generally true:

• Na autostradzie trzeba uważać...
• ...gdy prędkość rośnie.

This is like English:

• You need to be careful when speed increases.

It does not mean only right now. It means in general, whenever that happens.

Because that part is an added explanatory element, and it also contains a subordinate clause introduced by gdy.

Polish uses commas quite regularly before subordinate clauses, and gdy often triggers one.

So the comma helps separate:

• the main statement: Na autostradzie trzeba uważać bardziej niż w mieście
• the added qualification: zwłaszcza gdy prędkość rośnie

Yes, Polish word order is fairly flexible, though the original sounds very natural.

For example, you could also say:

• Trzeba bardziej uważać na autostradzie niż w mieście, zwłaszcza gdy prędkość rośnie.

That still means essentially the same thing.

The original version puts Na autostradzie first, which gives it a bit more focus, as if starting with the setting:

• On the motorway, you need to be more careful...

So the original word order is natural and slightly emphasizes the location.

A few words may be challenging for English speakers:

• trzeba: the beginning trz- is a consonant cluster; roughly like tsh
  • eba
• uważać: the ż sounds like the s in measure
• prędkość: contains nasal ę and the cluster -dkość
• rośnie: ś is a soft sh-like sound, and nie is pronounced as one smooth syllable group

A rough pronunciation guide:

• Na autostradzie → nah ow-toh-STRAH-djye
• trzeba uważać → TSHEH-bah oo-VAH-zhatch
• bardziej niż w mieście → BAR-djyei nizh f MYESH-chye
• zwłaszcza gdy prędkość rośnie → ZVWA-shcha gdi PREND-kosht ROSH-nye

These are only approximations, but they can help at first.