Learn Hardcore Croatian: Naša udruga za okoliš svake subote volontira u parku i oko rijeke. - Our association for the environment volunteers every Saturday in the park and around the river.

svaka subota – nominative; would sound like a subject (“every Saturday is …”)
svake subote – genitive; used as an adverbial phrase meaning “every Saturday” / “on Saturdays”

This “genitive of time” pattern is common:

svakog dana – every day
svake zime – every winter
svakog jutra – every morning

So svake subote functions like an adverb: “(on) every Saturday”.

What verb is volontira, and what does its form tell us?

Volontira is the 3rd person singular present tense of the verb volontirati (“to volunteer”).

Infinitive: volontirati
3rd sg. present: on/ona/ono volontira – “he/she/it volunteers”

Because the subject is Naša udruga za okoliš (third person singular), the verb must also be third person singular: udruga volontira – “the association volunteers”.

The present tense here expresses a habitual action, so the whole sentence means:
“Our environmental association volunteers (regularly) every Saturday …”

Is there a difference between volontira and saying “works as a volunteer”?

Yes, but it’s subtle.

volontirati – “to volunteer” (to do unpaid work)
raditi kao volonter – “to work as a volunteer”

In practice, in this sentence:

Naša udruga … volontira – natural, short, standard
Naša udruga … radi kao volonter – grammatically OK, but sounds odd, because an association doesn’t usually “work as a volunteer”; people do.

For people, both are possible:

Volontiram u parku. – I volunteer in the park.
Radim kao volonter u parku. – I work as a volunteer in the park.

Here volontira is the best choice.

Why is it u parku and not u park?

The preposition u can take accusative or locative, depending on meaning.

u + accusative – movement into something
- Idem u park. – I’m going to the park.
u + locative – location in/inside something
- Jesam u parku. – I am in the park.

In volontira u parku, we’re talking about the location of the volunteering, not movement, so park must be in the locative case: parku.

Why is it oko rijeke and what case is rijeke?

The preposition oko (“around”) normally takes the genitive case.

rijeka – river (nominative singular)
rijeke – genitive singular (among other forms)

So:

oko rijeke = “around the river” (physically, in the area surrounding the river).

This is different from English “about the river” (as a topic); for “about” (topic), Croatian usually uses o + locative:

pričati o rijeci – to talk about the river (topic)

Does oko ever mean “about” (as in “about 10 people”)?

Yes, oko can also mean “approximately” when used with numbers:

oko deset ljudi – about ten people
oko tri sata – about three hours

So oko has two common uses:

spatial: around + genitive – oko rijeke (around the river)
approximate number: about + number – oko deset ljudi (about ten people)

In your sentence, it’s clearly the spatial meaning: “around the river”.

How does gender agreement work in naša udruga?

Udruga is feminine in Croatian.
Possessive adjectives have to agree in gender, number, and case with the noun they modify.

masculine: naš prijatelj – our (male) friend
feminine: naša udruga – our association
neuter: naše selo – our village

So you must say naša udruga, not naš udruga.

Could the word order be different, like Svake subote naša udruga za okoliš volontira…?

Yes. Croatian word order is quite flexible, especially for adverbials like time expressions.

All of these are grammatically correct and natural:

Naša udruga za okoliš svake subote volontira u parku i oko rijeke.
Svake subote naša udruga za okoliš volontira u parku i oko rijeke.
Naša udruga za okoliš volontira svake subote u parku i oko rijeke.

The differences are mostly about emphasis:

Starting with Svake subote emphasizes how often/when.
Starting with Naša udruga… emphasizes who is doing it.

Can I omit naša or za okoliš and still be correct?

Grammatically, yes, but the meaning changes.

Udruga za okoliš svake subote volontira…
– “The environmental association volunteers every Saturday …”
You’re no longer specifying that it’s our association.
Naša udruga svake subote volontira…
– “Our association volunteers every Saturday …”
This doesn’t say what type of association it is; it could be a sports club, cultural group, etc.
Full version: Naša udruga za okoliš svake subote volontira…
– Specifies both whose association and what kind (environmental).

What’s the difference between udruga and organizacija (or društvo)?

All three can translate as “organization,” but usage differs:

udruga – very common for non-profit associations, NGOs, clubs
- udruga za okoliš – environmental association
- sportska udruga – sports association
organizacija – more general, often more formal or large-scale
- međunarodna organizacija – international organization
- nevladina organizacija (NVO) – non-governmental organization (NGO)
društvo – can mean society, club, company, group of people
- planinarsko društvo – mountaineering club
- društvo prijatelja knjige – society of friends of the book

In your sentence, udruga za okoliš is the most natural and typical expression.

Is the present tense here describing something happening right now, or a regular activity?

In Croatian, the present tense is used both for:

right now actions:
- Sad volontiram. – I’m volunteering (right now).
habitual / regular actions:
- Svake subote volontiram. – I volunteer every Saturday.

Because your sentence has svake subote (“every Saturday”), the present tense clearly describes a regular, repeated activity, not just today.

How would the main nouns in this sentence look in the plural?

Here are the nominative plural forms:

udruga (association) → udruge
okoliš (environment) → okoliši (rarely used in plural in practice)
park (park) → parkovi
rijeka (river) → rijeke
subota (Saturday) → subote

Example:

Naše udruge za okoliš svake subote volontiraju u parkovima i oko rijeka.
– Our environmental associations volunteer every Saturday in parks and around rivers.