Projekcijska oprema i ozvučenje su spremni.

Because the subject is made of two things joined by i (and):

• projekcijska oprema
• ozvučenje

Even though each noun is singular by itself, together they form a compound subject, so Croatian uses the plural verb:

• je = is
• su = are

So:

• Projekcijska oprema je spremna. = The projection equipment is ready.
• Ozvučenje je spremno. = The sound system is ready.
• Projekcijska oprema i ozvučenje su spremni. = The projection equipment and the sound system are ready.

For the same reason as su: the sentence has a compound subject.

The adjective spreman (ready) must agree with the subject. Since there are two coordinated nouns, Croatian uses a plural adjective:

• spreman = masculine singular
• spremna = feminine singular
• spremno = neuter singular
• spremni = masculine plural
• spremne = feminine plural
• spremna = neuter plural

Here the speaker is talking about two items together, so singular forms are not possible.

This is a very common learner question.

The two nouns have different genders:

• oprema is feminine
• ozvučenje is neuter

When Croatian combines nouns of different genders into one plural subject, the predicate adjective often defaults to masculine plural. That is why you get:

• su spremni

So even though there is no masculine singular noun here, masculine plural works as the default mixed-gender plural form.

The genders are:

• oprema = feminine
• ozvučenje = neuter

You can often see this from the endings and from agreement:

• projekcijska oprema
  The adjective projekcijska ends in -a, matching a feminine singular noun.

• ozvučenje
  The ending -e in the dictionary form is typical of many neuter nouns.

That is also why, if they appeared separately, you would say:

• oprema je spremna
• ozvučenje je spremno

Because it agrees with oprema.

Croatian adjectives change form to match the noun in gender, number, and case. Here:

• oprema is feminine singular
• the sentence uses the nominative case
• so the adjective is projekcijska

Compare:

• projekcijska oprema = projection equipment
• projekcijsko platno = projection screen
  (platno is neuter, so the adjective changes)
• projekcijski uređaj = projector / projection device
  (uređaj is masculine, so the adjective changes again)

Ozvučenje usually refers to sound equipment, sound reinforcement, or the audio/sound system, depending on context.

In event or technical contexts, it often means the equipment used for sound at a venue, such as speakers, microphones, amplifiers, and related audio setup.

So in this sentence, ozvučenje is not just sound in the abstract. It means something more like:

• the sound system
• the audio equipment
• the PA setup

Oprema is usually a collective noun, much like English equipment.

That means it is grammatically singular, but it can refer to a whole set of items. So:

• oprema je nova = the equipment is new
• not usually something like opreme su nove to mean multiple pieces of equipment in the ordinary sense

In your sentence, projekcijska oprema can include several physical items, but grammatically it is still treated as singular.

Because Croatian has no articles.

English distinguishes between:

• the projection equipment
• a projection system

Croatian does not use words like the or a/an. The meaning is understood from context.

So Projekcijska oprema i ozvučenje su spremni could mean:

• The projection equipment and sound system are ready
• or, depending on context, simply Projection equipment and sound system are ready

Usually the situation makes it clear.

No, Croatian word order is fairly flexible, although some orders sound more natural than others.

The neutral order here is:

• Projekcijska oprema i ozvučenje su spremni.

But you may also hear:

• Spremni su projekcijska oprema i ozvučenje.

That version puts more focus on spremni (ready). The original sentence is the most straightforward, neutral way to say it.

In standard Croatian, in a normal present-tense statement like this, no. You need the verb:

• Projekcijska oprema i ozvučenje su spremni.

Leaving out su would sound incomplete or nonstandard in ordinary prose.

Croatian does sometimes omit forms of to be in certain styles or structures, but not in a simple sentence like this one.

They are in the nominative case, because they are the subject of the sentence.

You can tell from the structure:

• Projekcijska oprema i ozvučenje = the things being talked about
• su spremni = are ready

So this is a basic subject + verb + predicate adjective sentence, and the subject appears in the nominative.

Here it is the short form, which is the normal form used in predicate position after to be.

So Croatian says:

• Oprema je spremna.
• Ozvučenje je spremno.
• Oprema i ozvučenje su spremni.

This is the usual way to say is/are ready.

Learners often notice that some Croatian adjectives have longer and shorter forms, but with spreman in this kind of sentence, the short predicative form is the normal one.

Then both the verb and the adjective would become singular, and the adjective would match the noun’s gender.

For example:

• Projekcijska oprema je spremna.
  feminine singular

• Ozvučenje je spremno.
  neuter singular

With both together:

• Projekcijska oprema i ozvučenje su spremni.
  plural, with masculine plural as the mixed-gender default

This is a good sentence for seeing how agreement works in Croatian.