Soba je svijetla kad je prozor otvoren.

Because there are really two clauses, and each one has its own form of to be:

• Soba je svijetla = The room is bright
• kad je prozor otvoren = when the window is open

So the first je goes with soba, and the second je goes with prozor.

Croatian adjectives change to match the noun's gender, number, and case.

Here, soba means room, and soba is a feminine singular noun. So the adjective must also be feminine singular:

• masculine: svijetao
• feminine: svijetla
• neuter: svijetlo

So:

• soba je svijetla = correct
• prozor je svijetao = masculine version
• mjesto je svijetlo = neuter version

For the same reason: the adjective must agree with the noun.

Prozor is a masculine singular noun, so the adjective is masculine singular too:

• prozor je otvoren = the window is open

If the noun were feminine, you would use otvorena:

• vrata su otvorena = the door is open in some contexts, though vrata is actually plural in form
• kutija je otvorena = the box is open

Yes. Kad means when, and kada is a longer form with the same meaning.

Both are common:

• Kad je prozor otvoren...
• Kada je prozor otvoren...

In everyday speech, kad is often a little more conversational and frequent.

Both are in the nominative because they are the subjects of their clauses:

• Soba je svijetla
• Prozor je otvoren

After biti (to be) in this kind of sentence, Croatian often uses an adjective to describe the subject, and the subject stays in the nominative.

In this sentence, it works like an adjective meaning open.

Croatian often uses forms like otvoren, zatvoren, umoran, spreman, etc. after biti to describe a state:

• Prozor je otvoren = The window is open
• Vrata su zatvorena = The door is closed / The doors are closed

Historically these forms are related to participles, but for a learner, it is most useful to think of them here as descriptive forms used like adjectives.

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

So:

• soba can mean a room or the room
• prozor can mean a window or the window

The exact meaning depends on context. In your sentence, English naturally uses the room and the window, but Croatian does not need separate words for that.

Yes. Croatian word order is more flexible than English.

You can also say:

• Kad je prozor otvoren, soba je svijetla.

This means the same thing: When the window is open, the room is bright.

The original version starts with Soba je svijetla, so it puts slightly more focus on the room being bright.
The version starting with Kad je prozor otvoren puts the time/condition first.

It can often be understood as a general whenever-type meaning in a sentence like this.

So Soba je svijetla kad je prozor otvoren can mean:

• The room is bright when the window is open
• or more naturally in context, The room is bright whenever the window is open

Croatian kad can express both a specific when and a habitual/general when, depending on context.

Sometimes, but the meaning changes a little.

• kad = when
• ako = if

So:

• Soba je svijetla kad je prozor otvoren = The room is bright when the window is open
• Soba je svijetla ako je prozor otvoren = The room is bright if the window is open

In many real situations the meanings overlap, but kad sounds more like a normal state or repeated situation, while ako sounds more conditional.

Yes. Both clauses are in the present tense:

• je = is
• je = is

Croatian often uses the present tense for general truths, regular situations, and descriptions, just like English does here.

Here it means bright, not light-colored.

The adjective svijetao / svijetla / svijetlo can sometimes relate to lightness in different senses, but in the sentence:

• Soba je svijetla kad je prozor otvoren

the context clearly means the room has a lot of light, so bright is the natural interpretation.

A rough guide is:

• svi-jet-la

Things that may feel unusual for an English speaker:

• sv comes together at the start
• ije is part of the standard Croatian spelling here
• tl in svijetla can feel like a tight consonant cluster

Try saying it slowly first:

• svi-jet-la

Then more smoothly:

• svijetla

You do not need to insert extra vowel sounds between the consonants.

Yes, it is natural and grammatical.

A native speaker might also say things like:

• Kad je prozor otvoren, soba je svijetla.
• Soba je svijetla kada je prozor otvoren.

All of these are normal. The differences are mainly in style, rhythm, or focus, not basic meaning.