Kad je roleta otvorena, soba je svjetlija.

Kad means when here.

Yes, kad and kada both mean when. In many everyday sentences, they are interchangeable. Kad is often a bit more common in speech and informal style, while kada can sound slightly more formal or more emphatic.

So you could also say:

Kada je roleta otvorena, soba je svjetlija.

with essentially the same meaning.

Because the sentence has two clauses, and each clause needs its own verb:

• Kad je roleta otvorena = When the blind/shutter is open
• soba je svjetlija = the room is brighter

In both clauses, je is the 3rd person singular present form of biti (to be), meaning is.

So literally, the structure is:

• When + is + the blind + open
• the room + is + brighter

Because otvorena agrees with roleta, and roleta is a feminine singular noun.

In Croatian, adjectives and adjective-like forms must agree with the noun in gender, number, and case.

• roleta = feminine singular
• therefore: otvorena = feminine singular

Compare:

• prozor je otvoren = the window is open
  (prozor is masculine)
• vrata su otvorena = the door / doors are open
  (vrata is plural in form)

So otvorena is used because it matches roleta.

It is originally a participle form, but in this sentence it functions like an adjective in a predicate.

So je otvorena means is open.

For a learner, the most useful way to understand it is simply:

• otvoren / otvorena / otvoreno = open

In sentences like this, Croatian often uses biti + a participle/adjectival form where English also uses to be + adjective:

• Prozor je zatvoren. = The window is closed.
• Roleta je otvorena. = The blind is open.

Svjetlija is the comparative form, meaning brighter.

The basic adjective is:

• svijetao / svijetla / svijetlo = bright, light

The comparative is:

• svjetliji / svjetlija / svjetlije = brighter

Here it is svjetlija because it agrees with soba, which is feminine singular.

So:

• soba = feminine singular
• therefore: svjetlija = feminine singular comparative

That is why the sentence says soba je svjetlija = the room is brighter.

Because Croatian does not have articles like English a/an and the.

So:

• roleta can mean a blind, the blind
• soba can mean a room, the room

The exact meaning depends on context.

In this sentence, English naturally uses the room and the blind/shutter, but Croatian does not need separate words for that.

Both are in the nominative case because they are the subjects of their clauses.

• roleta is the subject of je otvorena
• soba is the subject of je svjetlija

So:

• Kad je roleta otvorena → roleta = nominative
• soba je svjetlija → soba = nominative

This is a useful thing to notice: even though the sentence is fairly simple, it actually contains two mini-sentences, each with its own subject.

Because Kad je roleta otvorena is a subordinate clause placed before the main clause.

Croatian normally uses a comma to separate that kind of introductory clause from the main clause:

• Kad je roleta otvorena, soba je svjetlija.

This is similar to English:

• When the blind is open, the room is brighter.

If the order is reversed, you can also say:

• Soba je svjetlija kad je roleta otvorena.

In that version, there is usually no comma.

Yes. A very natural alternative is:

Soba je svjetlija kad je roleta otvorena.

This means the same thing: The room is brighter when the blind is open.

The difference is mainly emphasis:

• Kad je roleta otvorena, soba je svjetlija.
  starts by setting the condition/time frame
• Soba je svjetlija kad je roleta otvorena.
  starts with the main point: the room is brighter

However, you cannot move words completely freely. In particular, je is a clitic, and Croatian clitics tend to appear near the beginning of their clause, often in second position.

Not exactly.

Roleta usually means a blind or roller shutter, depending on region and context. It is something used to cover a window, but it is not the same as a fabric curtain.

So:

• roleta = blind / roller blind / shutter
• zavjesa = curtain
• žaluzina = venetian blind / slatted blind

If your translation says blind, that is a good general understanding.

It can often suggest a general truth, something close to whenever.

So this sentence can be understood as:

• When the blind is open, the room is brighter
• or more naturally in some contexts: Whenever the blind is open, the room is brighter

Croatian kad often works this way in general statements. The exact nuance comes from context rather than a special verb form.

Because svjetlija is part of the predicate describing soba.

In the clause:

• soba je svjetlija

the adjective/comparative tells us something about soba, so it must agree with soba:

• soba = feminine singular
• svjetlija = feminine singular comparative

This is the same agreement pattern you see in many Croatian sentences:

• Kuća je velika. = The house is big.
• Soba je svjetlija. = The room is brighter.

So whenever an adjective describes the subject after to be, it usually matches that subject.