Kad već počinje premijera, redar provjerava gledalište.

Both kad and kada mean when.

• kad is the shorter, very common everyday form.
• kada is a bit more formal or emphatic.

In this sentence, kad is completely natural. You could also say:

• Kada već počinje premijera, redar provjerava gledalište.

That would mean the same thing.

Već usually means already.

In this sentence, it adds a sense like:

• already
• by now
• sometimes even since / now that, depending on context

So Kad već počinje premijera... suggests something like when the premiere is already starting or now that the premiere is starting already.

It gives a slight feeling that the event is underway or that there is some urgency.

This is mainly about aspect.

• počinjati / počinje = imperfective → ongoing, in progress, repeated
• početi / počne = perfective → completed start, one whole event

Here, počinje fits well because the sentence describes the situation as it is unfolding: the premiere is beginning.

Also, in Croatian, the present tense of a perfective verb often refers to the future, so počne can sound more like will start in many contexts.

So:

• premijera počinje = the premiere is beginning / starts
• premijera počne = often more like the premiere starts/will start as a single completed onset

Premijera is in the nominative singular.

It is the subject of počinje:

• premijera počinje = the premiere is beginning

That is why it is in the basic dictionary form premijera.

Redar is a masculine noun. It usually means someone responsible for order or supervision in a public space.

Depending on context, it can be translated as:

• usher
• attendant
• steward
• security/order staff

In a theater or cinema context, usher is often the most natural English translation.

Here gledalište is in the accusative singular because it is the direct object of provjerava:

• redar provjerava gledalište = the usher checks the auditorium

The reason it looks unchanged is that gledalište is a neuter noun, and for many neuter nouns:

• nominative singular = accusative singular

So:

• nominative: gledalište
• accusative: gledalište

Same form, different function.

Usually gledalište means the auditorium, seating area, or the part of the theater where the spectators sit.

It does not normally mean the people themselves.
If you want to say the audience as people, you would more likely use:

• publika

So:

• redar provjerava gledalište = checking the hall/seating area
• redar provjerava publiku = checking the audience

Croatian word order is more flexible than English word order.

Both are possible:

• Kad već počinje premijera, redar provjerava gledalište.
• Kad premijera već počinje, redar provjerava gledalište.

The version with počinje premijera can sound a little more dynamic or stylistically natural in context. Croatian often moves words around for rhythm, emphasis, or information flow.

English speakers often expect one fixed order, but Croatian allows more variation as long as the case endings and verb forms make the roles clear.

Because Kad već počinje premijera is a subordinate clause, and it comes before the main clause.

So the structure is:

• subordinate clause: Kad već počinje premijera
• main clause: redar provjerava gledalište

In Croatian, it is standard to separate that with a comma:

• Kad već počinje premijera, redar provjerava gledalište.

This is similar to English:

• When the premiere begins, the usher checks the auditorium.

Croatian often uses the present tense to describe actions happening:

• right now
• regularly
• as part of a narration

Here the two present-tense verbs show two actions happening at the same time:

• počinje = the premiere is beginning
• provjerava = the usher is checking

English may sometimes prefer a progressive form, but Croatian simple present often covers that meaning.

So provjerava can mean:

• checks
• is checking

depending on context.

Because Croatian has no articles.

English uses:

• the premiere
• the usher
• the auditorium

Croatian simply says:

• premijera
• redar
• gledalište

Definiteness is understood from context, word order, or the situation. This is very normal in Croatian.

Yes. Depending on context, kad can sometimes feel a bit like:

• when
• once
• now that
• since

Because of već, the sentence can carry a nuance like:

• Now that the premiere is already starting, the usher checks the auditorium.

So even though the basic meaning is when, the full feeling can be slightly broader than a simple time expression.

Yes, provjerava is imperfective.

That matters because imperfective verbs are often used for:

• ongoing actions
• repeated actions
• background actions
• general processes

So redar provjerava gledalište presents the checking as an activity in progress, not as one completed result.

Compare:

• provjerava = is checking / checks
• provjeri = checks once / will check / completes the check, depending on context

In this sentence, the imperfective form fits well because the usher’s action is shown as part of the unfolding situation.