Oni su protiv smeća u parku.

Oni means they and is used when:

• the group is all male, or
• the group is mixed (at least one male present).

Other forms of they in Croatian:

• one – used for:
  • groups of only women: One su učiteljice. (They are teachers.)
  • plural non‑human feminine nouns.
• ona – used for:
  • plural neuter nouns, usually things: Ona su djeca. (They are children.)

So in your sentence, oni suggests either “all men” or “mixed group”. If it were clearly all women, you would say:

• One su protiv smeća u parku.

You can drop oni, but you cannot start the sentence with su.

Croatian often omits subject pronouns because the verb ending already shows the person and number. So these are natural:

• Oni su protiv smeća u parku. – neutral word order.
• Protiv su smeća u parku. – stressing protiv (“they are against trash in the park”).
• Protiv smeća u parku su. – less common, possible with the right intonation/context.

But Su protiv smeća u parku is wrong, because su is a clitic and must stand in (roughly) second position, not at the very beginning of the sentence.

So yes, you can drop oni, but keep su in second position: Protiv su smeća u parku.

Su is the 3rd person plural present tense of the verb biti (to be).

Present tense of biti:

• ja sam – I am
• ti si – you are (singular, informal)
• on/ona/ono je – he/she/it is
• mi smo – we are
• vi ste – you are (plural or polite singular)
• oni/one/ona su – they are

In your sentence, the subject is oni (they), so the verb must be su:

• Oni su protiv smeća u parku. – They are against trash in the park.

Protiv means against (in the sense of opposing something).

In grammar terms, protiv always governs the genitive case. That’s why smeće changes to smeća:

• nominative: smeće (trash)
• genitive: smeća

Some examples:

• Oni su protiv rata. – They are against the war.
• Protiv vlade su. – They are against the government.
• Protiv pušenja sam. – I am against smoking.

So the pattern is: protiv + genitive.

The base form is smeće (nominative), meaning trash / garbage.

Because it comes after protiv, and protiv requires the genitive, it changes to smeća:

• nominative: smeće – Smeće je svuda. (Trash is everywhere.)
• genitive: smeća – Protiv smeća su. (They are against trash.)

So smeća here is simply smeće in the genitive case.

The preposition u can take either:

• accusative – when there is movement into something:
  
  • Idem u park. – I am going to the park.
• locative – when it is location / position in something:
  
  • Šetaju u parku. – They are walking in the park.

Your sentence describes being against trash in a place (no movement), so u must take the locative case:

• park (nominative) → parku (locative): u parku – in the park.

So u parku = in the park, while u park = into the park.

Formally, for many masculine nouns like park, the dative and locative singular have the same ending (-u):

• dative sg: parku – Približavam se parku. (I am approaching the park.)
• locative sg: parku – Sjedi u parku. (He is sitting in the park.)

In your sentence, the preposition u with the meaning in (location) requires the locative, so:

• u parku is locative here.

We know it is locative because of the preposition and the meaning (static location, not “to the park” or “towards the park”).

Croatian does not have articles like the or a/an. The phrase u parku is neutral and can correspond to:

• in the park
• in a park

Which one you choose in English depends on the broader context:

• If the park is already known/specified, you translate the:
  
  • Govore o parku u našem kvartu. Oni su protiv smeća u parku.
    → They are talking about the park in our neighborhood. They are against trash in the park.
• If it’s just any park, you can translate a:
  
  • Oni su protiv smeća u parku.
    → They are against trash in a park / in parks.

Croatian leaves this implicit; English forces you to pick the or a based on context.

Yes, Croatian word order is quite flexible, and different orders slightly change emphasis rather than grammar.

All of these can be correct, depending on context and intonation:

• Oni su protiv smeća u parku.
  – neutral; focus on what they are against.
• Oni su u parku protiv smeća.
  – slightly more focus on location (in the park they are against trash, maybe not elsewhere).
• U parku su oni protiv smeća.
  – stressing u parku and oni (e.g. In the park, it is they who are against trash).
• Protiv smeća u parku su oni.
  – highlighting oni as the ones who are against trash in the park (contrast with other groups).

The basic meaning stays the same, but stress and nuance can shift with word order.

Both express a negative attitude, but the nuance is different:

• Oni su protiv smeća u parku.
  – literally They are against trash in the park.
  – implies opposition, possibly an active stance (campaigning, protesting, organizing cleanups, etc.).
• Oni ne vole smeće u parku.
  – They don’t like trash in the park.
  – weaker; describes dislike, not necessarily active opposition.

So protiv sounds stronger and more formal, often used in discussions of rules, policies, and opinions:

• Građani su protiv smeća u parku. – The citizens are against trash in the park.
• Građani ne vole smeće u parku. – The citizens don’t like trash in the park. (sounds more casual / descriptive).

Approximate pronunciation in IPA and English hints:

• smeća – /ˈsmɛt͡ɕa/

  • sm as in smell
  • e as in bet
  • ć is a soft ch sound, like a slightly softened tch in “cheap”, but more “palatal”
  • a as in father
  • stress on the first syllable: SME‑ća

• protiv – /ˈprɔtiv/ (often closer to /ˈprɔtiʋ/)

  • pro like pro in English but with a slightly more open o
  • t always hard (as in tap)
  • final v is often pronounced more like a w or u̯ in fast speech, but you can safely say a normal v
  • stress on the first syllable: PRO‑tiv