Learn Hardcore Croatian: Kad je zavjesa otvorena, zid izgleda širi, a soba je svjetlija. - When the curtain is open, the wall looks wider and the room is brighter.

Kad and kada mean the same thing. Kada is slightly more formal or careful; kad is the shorter, more colloquial everyday form. In this sentence you could say either:

Kad je zavjesa otvorena, …
Kada je zavjesa otvorena, …

Both are correct. The meaning and grammar are the same.

Why is it Kad je zavjesa otvorena and not something like Kad zavjesa je otvorena?

The neutral Croatian word order for to be + adjective is usually:

subject – auxiliary verb – adjective

So:

zavjesa je otvorena = the curtain is open

In a kad-clause, the same order is kept:

Kad je zavjesa otvorena, …

Putting je after zavjesa ( Kad zavjesa je otvorena ) sounds unnatural and is not standard word order. You can move the adjective:

Kad je otvorena zavjesa, …

This is grammatically correct, but it emphasizes otvorena a bit more and sounds less neutral than the original.

Why do we use je (the verb to be) in Kad je zavjesa otvorena? Could we just say Kad zavjesa otvorena?

You need the verb to be (biti) to form a normal sentence that describes a state:

zavjesa je otvorena = the curtain is open

Leaving out je (Kad zavjesa otvorena) is incorrect in standard Croatian. Unlike some Slavic languages that sometimes drop to be in the present tense, Croatian keeps it in sentences like this.

Why is it otvorena and not otvoreno or otvoren?

Otvorena is an adjective / passive participle that must agree with zavjesa in:

gender
number
case

Zavjesa is feminine, singular, nominative, so the adjective also has the feminine singular nominative ending:

otvoren – masculine singular (for zid, prozor, etc.)
otvorena – feminine singular (for zavjesa, vrata in some dialects, etc.)
otvoreno – neuter singular

So:

zid je otvoren – the wall is open (masc.)
zavjesa je otvorena – the curtain is open (fem.)

In this sentence, zavjesa is feminine, so otvorena is the correct form.

Is otvorena here an adjective or a verb? What exactly does it express?

Otvorena here functions as an adjective / passive participle that describes a state, not an action.

zavjesa je otvorena = the curtain is open (state)
zavjesa se otvara = the curtain is being opened / opens (ongoing action)
zavjesa se otvorila = the curtain opened (completed action)

In the sentence Kad je zavjesa otvorena, we are talking about the condition in which the rest of the sentence is true: When the curtain is in the state of being open...

Why is zavjesa feminine? How can I know the gender?

In Croatian every noun has a grammatical gender: masculine, feminine, or neuter.

Zavjesa (curtain) is feminine. A common pattern is:

Nouns ending in -a are often (not always) feminine:
zavjesa, kuća (house), soba (room), žena (woman)

You usually learn the gender together with the noun, but the ending -a is a strong hint that zavjesa is feminine. That’s why adjectives and participles describing it are feminine: otvorena, nova, bijela, etc.

Why does the sentence say zid izgleda širi and not zid je širi?

zid je širi = the wall is wider (a statement of fact)
zid izgleda širi = the wall looks/appears wider (how it seems)

The verb izgledati means to look / to appear. Using izgleda emphasizes that we are talking about appearance, not an absolute physical measurement.

Given the context (a curtain being open), it makes sense: the wall seems wider because more of it is visible or because the room looks more open.

Why is širi in the nominative case, instead of some other ending?

With verbs like biti (to be) and izgledati (to look/seem), the adjective that describes the subject is in the nominative, agreeing with the subject in gender and number.

zid izgleda širi
- zid – masculine singular nominative
- širi – masculine singular nominative to match zid

This is similar to English: The wall looks wider (adjective linked directly to the subject by a copular verb).

How is the comparative širi formed from širok? Why don’t we use više širok?

Croatian normally forms the comparative of most adjectives by changing the ending, not by adding više:

širok → širi = wide → wider
svijetao → svjetliji/svjetlija = bright → brighter
velik → veći = big → bigger

You usually do not say više širok for the basic comparative. Instead you use the synthetic form širi.

Više is used:

for some long adjectives or borrowed adjectives: više moderan
to mean more in a separate sense, e.g. više nego prije (more than before)

But here, širi is the normal, correct comparative.

Does širi agree with zid in gender and number?

Yes.

zid is masculine singular nominative
širi is the masculine singular nominative form of the comparative

If the subject changed, the adjective would change form:

zid izgleda širi – the wall looks wider (masc. sg.)
soba izgleda šira – the room looks wider (fem. sg.)
prozor izgleda širi – the window looks wider (masc. sg.)
vrata izgledaju šira – the doors look wider (neut. pl., special noun)

What is the difference between a and i? Why is it a soba je svjetlija and not i soba je svjetlija?

Both a and i can be translated as and, but they are used a bit differently:

i = simple addition (and, as well as)
a = often contrasts or slightly separates two ideas, something like and / while / whereas

In this sentence:

zid izgleda širi, a soba je svjetlija.

The speaker is presenting two related but slightly separate effects of the curtain being open:

the wall looks wider
the room is brighter

Using a lightly contrasts or distinguishes these two results instead of just listing them.

You could say:

zid izgleda širi i soba je svjetlija

This is understandable, but a sounds more natural here because it subtly separates the two consequences.

Why is it soba je svjetlija and not soba izgleda svjetlija like with the wall?

Both are possible:

soba je svjetlija – the room is brighter (treating it almost like an objective change)
soba izgleda svjetlija – the room looks/appears brighter (focusing on perception)

The author probably chose je svjetlija to state it more directly: opening the curtain actually makes the room brighter because more light enters.

With zid izgleda širi, it’s more obviously about illusion/appearance, so izgleda fits better.

How is svjetlija formed, and why is it svjetlija and not svjetliji?

The basic adjective is svijetao (bright, light). Its stem changes in comparison:

svijetao → svjetliji / svjetlija / svjetlije, etc.

Soba is feminine singular nominative, so the comparative must match:

svjetliji – masculine singular (for zid, prozor…)
svjetlija – feminine singular (for soba, kuća…)
svjetlije – neuter singular (for svjetlo, etc.)

So:

soba je svjetlija – the room is brighter (fem. sg.)

Why is there a comma after otvorena?

The sentence has:

a subordinate clause: Kad je zavjesa otvorena (When the curtain is open)
followed by the main clause: zid izgleda širi, a soba je svjetlija (the wall looks wider, and the room is brighter)

In Croatian, a subordinate clause introduced by kad/kada is normally separated from the main clause with a comma:

Kad je zavjesa otvorena, zid izgleda širi, a soba je svjetlija.

Is the present tense izgleda here about now, or is it more general? Could we say this as a general fact?

The present tense in Croatian, like in English, can express:

what is happening now, or
a general truth / repeated situation

Here, Kad je zavjesa otvorena, zid izgleda širi, a soba je svjetlija is best read as a general statement:

Whenever the curtain is open, the wall looks wider and the room is brighter.

So it’s not only about this moment, but about what usually / always happens under that condition.