Learn Hardcore Croatian: Zrak oko stadiona je ponekad zagađen zbog prometa. - The air around the stadium is sometimes polluted because of the traffic.

You would not normally use it in the plural to mean types of air or many airs, just like in English. Plural zraci exists, but that means rays (as in rays of light), not multiple airs. Context decides the meaning.

Why is it oko stadiona? I thought oko means eye. How does it mean around here?

Oko has two completely different meanings in Croatian, depending on context:

oko (noun) = eye
- oko – an eye
- oči – eyes
oko (preposition) = around / about / approximately
- When used as a preposition, it is followed by the genitive case:
  - oko stadiona – around the stadium
  - oko kuće – around the house

In your sentence, oko is a preposition, not a noun, so it means around, and it forces the word stadion into the genitive form stadiona.

Why is it stadiona and not just stadion?

Stadion is the base (nominative) form:

stadion – stadium (nominative)

The preposition oko always takes the genitive case. So stadion must change to its genitive form stadiona:

oko stadiona – around the stadium

This is a general pattern:

oko grada – around the city
oko kuće – around the house

So the -a in stadiona is simply the genitive singular ending for a masculine noun here, required by oko.

What is the role of je in je ponekad zagađen? Is it always necessary?

Je is the 3rd person singular form of the verb biti – to be:

on/ona/ono je – he/she/it is

In your sentence:

Zrak … je … zagađen. – literally: The air is polluted.

So je is necessary as the main verb is here.
If you remove it:

Zrak oko stadiona ponekad zagađen. – this is ungrammatical in standard Croatian.

You only drop je in a few specific contexts (e.g. some colloquial speech or certain subordinate clauses), but not in this simple main clause.

Why is it ponekad zagađen and not something like zagađen ponekad? Where does ponekad usually go?

Ponekad means sometimes and is an adverb. Croatian word order is flexible, but some positions are more natural.

In your sentence, common options are:

Zrak oko stadiona je ponekad zagađen.
Zrak oko stadiona je zagađen ponekad. – possible, but sounds less neutral and more stylistic/emphatic.
Ponekad je zrak oko stadiona zagađen. – also natural; here you emphasize sometimes.

The most typical, neutral variant in spoken language is what you have:

subject (Zrak oko stadiona) + verb (je) + adverb (ponekad) + complement/predicate (zagađen).

So ponekad usually stands close to the verb or the adjective/participle it modifies, but you can move it for emphasis.

What exactly is zagađen? Is it an adjective, or a verb form? Why not zagađeni or zagađena?

Zagađen is a passive participle of the verb zagaditi (to pollute), and it functions like an adjective meaning polluted / contaminated.

It must agree with the noun zrak in:

gender: masculine
number: singular
case: nominative (because it’s the predicate describing the subject)

So we get:

zrak je zagađen – the air is polluted (masc. sg.)

Other forms, for agreement with different nouns, would be:

zagađena voda – polluted water (feminine singular)
zagađeno more – polluted sea (neuter singular)
zagađeni gradovi – polluted cities (masculine plural)

In your sentence the subject is zrak (masc. sg.), so zagađen is the correct form.

Why is there no word for the in Zrak oko stadiona? How do you know it’s the air and not air in general?

Croatian has no articles like the or a/an. Context does all the work.

Zrak oko stadiona je ponekad zagađen can be translated as:

The air around the stadium is sometimes polluted, or
Air around the stadium is sometimes polluted,

depending on the context and what sounds natural in English.

If you want to emphasize general air in a more abstract or generic way, context and additional wording help, e.g.:

Zrak u gradovima je često zagađen. – Air in cities is often polluted. (The air in cities… is also fine.)

So you never add a separate word for the; you just interpret it from context.

What does prometa mean exactly? Is it traffic, transport, or something else? And why is it prometa and not promet?

The base noun is:

promet – usually traffic (movement of vehicles, people, goods), but sometimes commerce/turnover in other contexts.

In your sentence it clearly means traffic (on the roads).

The preposition zbog (because of / due to) requires the genitive case. So promet becomes prometa (genitive singular):

zbog prometa – because of the traffic

Other examples with zbog:

zbog kiše – because of the rain
zbog buke – because of the noise

So prometa is just promet in the genitive, required by zbog.

Why is it zbog prometa and not od prometa? What’s the difference between zbog and od here?

Both zbog and od can relate to cause or origin, but they’re not interchangeable.

zbog = because of / due to / on account of
- very common for expressing reason or cause:
  - Zrak je zagađen zbog prometa. – The air is polluted because of traffic.
  - Kasnim zbog gužve. – I’m late because of the crowd/traffic jam.
od = from / of / by, very broad meaning
- can sometimes indicate a cause (especially in set phrases like umrijeti od gladi – die of hunger),
- but zbog is the normal, straightforward choice for because of traffic.

Od prometa would sound strange here if you want to say “because of traffic”; zbog prometa is the natural collocation.

Could you also say Zrak je ponekad zagađen oko stadiona? Does moving oko stadiona change the meaning?

Yes, you can say:

Zrak je ponekad zagađen oko stadiona.

This is grammatically correct and would normally be understood with the same meaning:

The air is sometimes polluted around the stadium.

Differences are mostly about rhythm and focus:

Zrak oko stadiona je ponekad zagađen.
- Slightly more focus on the air around the stadium as a specific area.
Zrak je ponekad zagađen oko stadiona.
- Slightly more focus on the air in general, and then you narrow it down with around the stadium.

In everyday speech, both sound natural and would not confuse anyone.

Is promet the only word for traffic? I’ve seen saobraćaj somewhere—what’s the difference?

In standard Croatian, the normal word is:

promet – for traffic

Saobraćaj is more typical for Serbian (and Bosnian in many areas). In Croatia it is understood but felt as non-standard/Serbian.

So in Croatian you say:

gust promet – heavy traffic
promet u gradu – traffic in the city
zbog prometa – because of the traffic

For learning Croatian, you should stick to promet rather than saobraćaj.

How would I say The air around the stadium is very polluted because of heavy traffic using the same pattern?

You can extend the original sentence like this:

Zrak oko stadiona je jako zagađen zbog gustog prometa.

Breakdown:

Zrak oko stadiona – The air around the stadium
je – is
jako zagađen – very polluted (jako = very)
zbog gustog prometa – because of heavy traffic (gusti promet → gustog prometa in genitive after zbog)