Sporedna pozornica je otvorena.

Because pozornica is a feminine singular noun, and the adjective must agree with it in gender, number, and case.

• pozornica = feminine singular
• so the adjective is otvorena = feminine singular
• masculine singular would be otvoren
• neuter singular would be otvoreno

For example:

• Ulaz je otvoren. = The entrance is open.
• Vrata su otvorena. = The door / doors are open.
  
  • Here vrata is grammatically plural, so otvorena is plural as well.

Je is the 3rd person singular form of the verb biti = to be.

So here:

• pozornica je otvorena = the stage is open

It is the Croatian equivalent of is.

Some forms of biti in the present tense are:

• sam = am
• si = are
• je = is
• smo = are
• ste = are
• su = are

They are in the nominative singular.

This is because pozornica is the subject of the sentence, and subjects are normally in the nominative.

• pozornica = nominative singular
• sporedna matches it, so it is also nominative singular feminine

So the structure is:

• sporedna pozornica = the side/secondary stage
• je otvorena = is open

Croatian normally does not use articles like the or a/an.

So:

• pozornica can mean a stage or the stage
• the exact meaning depends on context

In your sentence, English would often use the because the speaker probably means a specific stage already known in the situation.

Croatian relies on:

• context
• word order
• sometimes demonstratives like ta, ova, ona

For example:

• Ova pozornica je otvorena. = This stage is open.
• Ta pozornica je otvorena. = That stage is open.

Sporedna means something like:

• secondary
• side
• auxiliary
• less main

In this sentence, sporedna pozornica is most naturally:

• the side stage
• the secondary stage

The exact best English wording depends on context. At a festival or event, it often means a smaller or less central stage compared with the main stage.

It comes from sporedan, which means secondary, incidental, or not main.

It can be understood both ways, depending on context.

In everyday Croatian, je otvorena often simply functions like is open in English, describing a state.

So:

• Pozornica je otvorena. can mean The stage is open.

Grammatically, otvorena is the past passive participle of otvoriti = to open, but in many sentences it behaves very much like an adjective.

Compare:

• Vrata su otvorena. = The door is open / The doors are open.
• Trgovina je otvorena. = The shop is open.

These usually describe the current state, not necessarily the action itself.

Yes. Croatian word order is more flexible than English word order.

The neutral, ordinary order here is:

• Sporedna pozornica je otvorena.

But you could also see:

• Otvorena je sporedna pozornica.
• Je otvorena sporedna pozornica.
  
  • this is much less neutral and depends strongly on style or context

Changing word order usually changes focus or emphasis, not the core meaning.

For example:

• Sporedna pozornica je otvorena. = neutral statement
• Otvorena je sporedna pozornica. = emphasis on open, or contrast with something else

Because sporedna agrees with pozornica in form.

Both are:

• feminine
• singular
• nominative

This agreement is a key feature of Croatian grammar. Adjectives usually match the noun they describe in:

• gender
• number
• case

So sporedna pozornica is one noun phrase: secondary/side stage.

A rough pronunciation is:

SPO-red-na po-ZOR-ni-tsa ye ot-VO-re-na

A few helpful notes:

• r is trilled or tapped
• j is pronounced like English y
• c is pronounced like ts
• stress is not usually marked in normal writing, but a learner-friendly approximation is:
  • SPO-red-na
  • po-ZOR-ni-ca
  • ot-VO-re-na

So je sounds like ye, not like English jee.

Yes. That would simply mean:

• The stage is open

Adding sporedna makes it more specific:

• Sporedna pozornica je otvorena = The side/secondary stage is open

So sporedna is just adding extra information about which stage you mean.

By itself in this kind of sentence, it usually means open, describing a state.

If you want to emphasize the action was opened, Croatian often uses context or a fuller passive-style expression. But in everyday usage, sentences like this are usually understood statively:

• Pozornica je otvorena. = The stage is open.

If you wanted an active sentence about someone opening it, you would more naturally say something like:

• Otvorili su pozornicu. = They opened the stage.

So in your sentence, the most natural interpretation is the state, not the action.