Vreća za spavanje i prostirka ostaju u garderobi.

Because the subject is a compound subject: vreća za spavanje i prostirka = the sleeping bag and the mat.

In Croatian, when two things are joined by i (and), the verb usually goes in the plural:

• Vreća ... ostaje = The sleeping bag stays
• Prostirka ostaje = The mat stays
• Vreća ... i prostirka ostaju = The sleeping bag and the mat stay

So ostaju is the 3rd person plural form of ostajati / ostati.

Literally, vreća za spavanje means bag for sleeping.

This is a very common Croatian pattern:

• za
  • verbal noun / noun = for ...
• vreća za spavanje = sleeping bag
• compare:
  • četkica za zube = toothbrush (literally brush for teeth)
  • stroj za pranje = machine for washing

So spavanje is a verbal noun meaning sleeping, and za spavanje means for sleeping.

Prostirka means a mat, pad, or groundsheet-like mat, depending on context.

In outdoor or camping contexts, it often refers to the thing you put on the ground under or beside a sleeping bag. So in English, depending on the situation, it might be translated as:

• mat
• sleeping mat
• camping pad
• ground mat

It is a feminine noun.

Because u meaning in takes the locative case when it expresses location.

Here, garderoba is the dictionary form nominative singular, but after u meaning in, it becomes:

• garderoba → u garderobi

So:

• u garderobi = in the cloakroom / in the changing room / in the wardrobe area, depending on context

This -i ending is a normal locative singular ending for many feminine nouns in -a.

It depends on context. Garderoba can mean things like:

• cloakroom
• checkroom
• changing room
• wardrobe area

So u garderobi is not always literally in the wardrobe in the furniture sense. In many real-life situations, it means a place where clothes or personal items are kept.

Because Croatian has no articles like English a/an/the.

So:

• vreća can mean a bag or the bag
• prostirka can mean a mat or the mat
• garderoba can mean a cloakroom or the cloakroom

The exact meaning is understood from context.

So Vreća za spavanje i prostirka ostaju u garderobi could be understood as:

• The sleeping bag and the mat stay in the cloakroom
• or A sleeping bag and a mat stay in the cloakroom

In practice, context tells you which one is intended.

Yes, ostaju is present tense.

Verbally, it means stay / remain / are staying. But in Croatian, present tense can also be used in practical instructions or arrangements, especially when talking about what is supposed to happen.

So this sentence could mean:

• The sleeping bag and the mat remain in the cloakroom
• The sleeping bag and the mat stay in the cloakroom
• in context, even something like The sleeping bag and the mat are to be left in the cloakroom

So although it is grammatically present tense, the practical meaning may sound a bit like a rule or instruction.

The form ostaju comes from the imperfective verb ostajati.

Croatian verbs often come in aspect pairs:

• ostati = perfective, roughly to stay / remain / be left as a completed event
• ostajati = imperfective, roughly to be staying / to remain / to keep staying

Present-tense forms of ostajati are normal present tense forms, such as:

• ostajem
• ostaješ
• ostaje
• ostajemo
• ostajete
• ostaju

A learner may notice the perfective partner ostati, but perfective verbs generally do not give a normal present meaning in the same way.

Yes, that alternative word order is also possible.

Croatian word order is more flexible than English word order because case endings show grammatical relationships. The original sentence:

• Vreća za spavanje i prostirka ostaju u garderobi.

is a neutral, straightforward order: subject + verb + place.

But Croatian can move elements around for emphasis:

• U garderobi ostaju vreća za spavanje i prostirka.
  = emphasizes where they stay

The meaning stays basically the same, but the focus shifts a little.

A rough guide:

• vreća ≈ VRE-cha, but with a softer sound than English ch
• garderobi ≈ gar-de-RO-bi
• ž is like the s in measure
• ć is a soft Croatian consonant, somewhat between English t and ch, but softer than č

So:

• vreća: the ć is soft
• za spavanje: straightforward pronunciation
• garderobi: the r is trilled or tapped more than in English

A native English speaker often does best at first by hearing these sounds from native audio, because ć and č are difficult to master from spelling alone.

Yes. All three are feminine singular nouns in their basic dictionary form:

• vreća
• prostirka
• garderoba

You can often recognize feminine nouns because many of them end in -a, though that is not an absolute rule for every Croatian noun.

That helps explain forms like:

• u garderobi — locative singular of a feminine noun

In full, vreća za spavanje is the normal clear expression for sleeping bag.

In conversation, speakers may shorten things if the context is obvious, but for learners, the full phrase is the safest and most standard form to remember.

So if you are labeling equipment or giving instructions, vreća za spavanje is exactly the phrase you want.