Learn Hardcore Croatian: Moja prijateljica je volonterka u istoj udruzi, a njezin brat je volonter u azilu za životinje. - My friend is a volunteer female in the same association, and her brother is a volunteer male in an animal shelter.

If the friend is male, you would say moj prijatelj.
If you want to speak generically and do not care about gender, people still usually default to the masculine form prijatelj, but when you know the person’s gender, you normally use the matching form.

What is the difference between volonter and volonterka?

Again, this is a masculine–feminine pair:

volonter = male volunteer
volonterka = female volunteer

In the sentence:

Moja prijateljica je volonterka… → the friend is female → volonterka
…njezin brat je volonter… → the brother is male → volonter

Croatian often has this pattern:

student / studentica
učitelj / učiteljica
volonter / volonterka

What does moja mean, and why is it moja prijateljica and not moj prijateljica?

Moja means my and it agrees in gender and case with the noun it describes.

prijateljica is feminine singular, nominative.
The possessive moj (my) has to match that:
- masculine: moj prijatelj (my [male] friend)
- feminine: moja prijateljica (my [female] friend)
- neuter: moje dijete (my child)

So:

moja prijateljica = my female friend
saying moj prijateljica would be grammatically wrong, because moj is masculine and prijateljica is feminine.

What does njezin mean, and how is it different from njen or njegov?

njezin = her (belonging to a female person)
njegov = his (belonging to a male person)
njen is a shorter, more colloquial form of njezin; it is common in speech, but njezin is considered more standard and formal.

In the sentence:

njezin brat = her brother (the brother belongs to that female friend)

Examples:

Njezin pas = her dog (owner is female)
Njegov pas = his dog (owner is male)

You will often hear:

njen brat in everyday speech instead of njezin brat.

Why is it u istoj udruzi and not something like u ista udruga?

The phrase u istoj udruzi is in the locative case, used after u when talking about being in a place (not moving into it).

Nominative (dictionary form): ista udruga (the same association)
Locative singular feminine:
- ista → istoj
- udruga → udruzi

So:

u istoj udruzi ≈ in the same association

Both the adjective istoj and the noun udruzi show endings that mark feminine singular locative, and they must agree with each other.

Why does u sometimes use the locative (like u istoj udruzi, u azilu) and sometimes the accusative?

The preposition u can take two different cases, depending on meaning:

Locative = location, where something is (static)
- u istoj udruzi = in the same association
- u azilu = in the shelter
Accusative = direction, movement into somewhere
- Idem u udrugu. = I am going into the association.
- Vode psa u azil. = They are taking the dog to the shelter.

In this sentence there is no movement; it only describes where they are volunteers, so u is followed by the locative.

What exactly does udruga mean? Is it the same as organizacija?

Udruga is usually translated as association, non-profit organisation, or NGO, especially in the Croatian legal context. It typically refers to:

a registered group (club, society, NGO)
formed around some common interest or goal.

Organizacija is a broader word, like organization in English. It can mean any kind of organized structure, including companies, institutions, etc.

In everyday speech:

udruga often suggests a non-profit / civic association.
organizacija is more general.

What does the conjunction a mean here, and how is it different from i or ali?

Croatian has several coordinating conjunctions:

i = and (simply adds things)
ali = but (expresses contrast)
a = and / but, often a mild contrast or just shifting to another person/topic

In the sentence:

…, a njezin brat je volonter…

a connects two related facts but also signals a slight contrast or change of focus:

Her friend volunteers in one place,
while her brother volunteers in another.

You could use i here:

…, i njezin brat je volonter…

That sounds more like simple addition: and her brother is also a volunteer.
With a, it subtly feels like she is in one organization, while he is (instead) in an animal shelter.

Why is je placed after Moja prijateljica and after njezin brat, and can it move?

Je is the 3rd person singular of biti (to be): he/she/it is.

In Croatian, je is a clitic (a short unstressed word) and very often appears in the second position in the clause. Typical pattern:

[First stressed word or phrase] + je + [rest]

So:

Moja prijateljica je volonterka…
…, a njezin brat je volonter…

You usually cannot freely move je to the end like in English:

✗ Moja prijateljica volonterka je (sounds wrong in neutral speech)

The most natural position for je here is exactly where it appears: right after the subject phrase.

Why does Croatian not use articles like a or the in this sentence?

Croatian does not have articles like English a/an or the. Nouns appear without them:

prijateljica can mean a friend or the friend, depending on context.
volonterka can mean a volunteer or the volunteer.

The specificity is understood from context, word order, and sometimes from other words (like ovaj, taj, onaj = this/that).

So:

Moja prijateljica je volonterka…
- can correspond to My friend is a volunteer…
- or My friend is the volunteer…, depending on what situation you are talking about.

Croatian speakers simply don’t need articles to express this difference most of the time.

What case is za životinje, and why does it look like the dictionary form životinje?

Za takes the accusative case, so životinje here is accusative plural.

Dictionary form (nominative plural): životinje = animals
Accusative plural (after za): also životinje

For many feminine nouns ending in -a, the nominative plural and accusative plural forms are identical:

Nom. pl.: životinje
Acc. pl.: životinje

Only the function in the sentence and the preposition za tell you that it is accusative:

u azilu za životinje ≈ in an animal shelter (literally: in a shelter for animals).

Could you leave out moja or njezin and just say Prijateljica je volonterka… or …a brat je volonter…?

Grammatically, yes, you could say:

Prijateljica je volonterka u istoj udruzi.
…, a brat je volonter u azilu za životinje.

But the meaning changes:

moja prijateljica = my (female) friend – explicitly yours.
just prijateljica = a (female) friend (of someone, probably you, but less explicit).

For brat (brother), Croatian very often omits the possessive when it is clear from context whose brother it is, especially in close family contexts:

Brat je volonter u azilu.
→ would usually be understood as my brother in the right context.

In the original sentence, moja and njezin make it very clear that:

the friend belongs to the speaker,
the brother belongs to that friend.