Ozvučenje i projekcijska oprema rade dobro.

Because the subject is made of two nouns joined by i = and.

In Croatian, two singular nouns connected this way normally take a plural verb:

• Ozvučenje radi. = The sound system works.
• Projekcijska oprema radi. = The projection equipment works.
• Ozvučenje i projekcijska oprema rade. = The sound system and the projection equipment work.

So rade is plural because the sentence has a compound subject.

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

You can see the structure like this:

• ozvučenje = nominative singular neuter
• projekcijska oprema = nominative singular feminine
• rade = 3rd person plural present
• dobro = adverb

So the sentence is built around who/what is doing the action: the sound system and the projection equipment.

Because the head noun is oprema, and oprema is grammatically singular in Croatian.

Even though equipment refers to a collection of things, Croatian usually treats oprema as a singular noun, just like English equipment. The adjective has to agree with that noun, so:

• oprema = feminine singular
• projekcijska = feminine singular adjective matching oprema

So projekcijska oprema is grammatically singular, even though it may refer to several pieces of equipment.

Ozvučenje usually refers to the sound system, audio setup, or sound reinforcement equipment in a room or venue.

It is not just the abstract idea of sound. In contexts like meetings, events, classrooms, or conference halls, ozvučenje often means the microphones, speakers, mixer, and related audio system as a whole.

So it behaves a bit like a collective/system noun.

Because dobro here is an adverb, not an adjective.

It modifies the verb rade:

• rade dobro = work well

If you used dobra or dobri, those would be adjective forms and would need to describe a noun, not the action.

Compare:

• Oprema je dobra. = The equipment is good.
  Here dobra is an adjective describing oprema.

• Oprema radi dobro. = The equipment works well.
  Here dobro is an adverb describing radi.

That distinction is very important in Croatian.

It is grammatically singular.

This is similar to English equipment, which is also usually treated as singular:

• Oprema je nova. = The equipment is new.
• Oprema radi. = The equipment works.

Even if the equipment includes many separate items, Croatian normally uses the singular noun oprema.

It can mean either, depending on context.

Croatian present tense often covers both:

• a general statement: they work well
• a current situation: they are working well

So without more context, Ozvučenje i projekcijska oprema rade dobro could mean:

• the sound and projection equipment generally function well
• the sound and projection equipment are functioning well right now

Croatian usually relies on context rather than forcing a separate continuous form like English does.

Yes. Croatian word order is fairly flexible.

This sentence is a neutral, natural order:

• Ozvučenje i projekcijska oprema rade dobro.

But you could also say:

• Projekcijska oprema i ozvučenje rade dobro.
• Ozvučenje i projekcijska oprema dobro rade.

These versions are still understandable, but the emphasis may shift slightly. The original version sounds very natural and neutral.

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

So Croatian simply says:

• ozvučenje
• projekcijska oprema

and context tells you whether English should use the, a, or no article at all.

That is why one Croatian sentence can correspond to several English possibilities, such as:

• The sound system and the projection equipment work well
• Sound and projection equipment work well

In standard Croatian, with two nouns joined by i, the expected verb is still plural: rade.

So this sentence should be:

• Ozvučenje i projekcijska oprema rade dobro.

Using radi would sound non-standard here, because the grammar follows the fact that there are two coordinated subject elements, even if together they belong to one overall technical setup.