Kad je velika vlaga u zraku, teže dišem.

Yes. Kad and kada both mean when.

• Kad is the shorter, very common everyday form.
• Kada is a bit fuller and can sound slightly more formal or more careful in style.

So both of these work:

• Kad je velika vlaga u zraku, teže dišem.
• Kada je velika vlaga u zraku, teže dišem.

In normal speech, kad is extremely common.

The comma separates the dependent clause from the main clause.

• Kad je velika vlaga u zraku = subordinate clause (When the humidity in the air is high)
• teže dišem = main clause (I breathe more heavily / with more difficulty)

Croatian normally uses a comma before or after this kind of time clause, depending on word order.

Compare:

• Kad je velika vlaga u zraku, teže dišem.
• Teže dišem kad je velika vlaga u zraku.

Both are correct. The first one starts with the when-clause, so the comma is especially expected there.

Vlaga is a feminine singular noun meaning humidity or moisture.

Because vlaga is feminine singular, the adjective must agree with it:

• velik = masculine singular
• velika = feminine singular
• veliko = neuter singular

So:

• velika vlaga = high humidity / literally big humidity

The phrase is in the nominative singular because it is the subject of je.

Yes, literally it looks like big humidity, but in Croatian this is a normal way to express high humidity.

Croatian often uses velik / velika / veliko where English uses high. So this is natural Croatian, not a mistake.

You may also hear related expressions, but velika vlaga is perfectly standard and natural.

Because u can take different cases depending on meaning.

Here it means in the air, expressing location, not movement. With location, u takes the locative case.

• nominative: zrak = air
• locative: u zraku = in the air

Compare:

• u zraku = in the air
• u zrak = into the air

So in this sentence, u zraku is correct because the humidity is located in the air.

Zraku is the locative singular of zrak.

The pattern here is:

• u + locative for location

So:

• zrak = air
• u zraku = in the air

This is a very common pattern in Croatian:

• u kući = in the house
• u gradu = in the city
• u zraku = in the air

Teže is the comparative form of the adverb teško.

• teško = heavily, hard, with difficulty
• teže = more heavily, harder, with more difficulty

So teže dišem means:

• I breathe with more difficulty
• I breathe harder
• I have more trouble breathing

In natural English, we often translate it more smoothly than word-for-word.

It is an adverb here, because it modifies the verb dišem (I breathe).

It tells us how the person breathes:

• dišem = I breathe
• teže dišem = I breathe more difficultly / I breathe harder

If it were an adjective, it would describe a noun and would need to agree with that noun. Here it does not describe a noun; it describes the action.

In Croatian, the subject pronoun is often omitted because the verb ending already shows the subject.

• dišem = I breathe

The ending -em tells you it is 1st person singular.

So:

• Teže dišem. = I breathe harder.
• Ja teže dišem. = also correct, but ja adds emphasis

You would use ja if you want contrast or emphasis, for example:

• On dobro diše, ali ja teže dišem.
• He breathes fine, but I breathe with more difficulty.

Dišem is present tense, first person singular, from the verb disati (to breathe).

So:

• dišem = I breathe / I am breathing

Croatian present tense can often cover both English simple present and present continuous, depending on context.

In this sentence, it has a general meaning:

• When the humidity is high, I breathe harder.

Yes, absolutely.

Both are natural:

• Kad je velika vlaga u zraku, teže dišem.
• Teže dišem kad je velika vlaga u zraku.

The meaning is basically the same, but the emphasis changes a little:

• starting with Kad je... emphasizes the condition first
• starting with Teže dišem... emphasizes the result first

Croatian word order is more flexible than English, but not completely free. These two versions are both standard and natural.

It is grammatically possible, but it is not the most natural order here.

The most neutral order is:

• velika vlaga

Putting the adjective after the noun, as in vlaga velika, can sound marked, poetic, or contrastive depending on context.

So for normal everyday Croatian, kad je velika vlaga u zraku is the best choice.

Ž is pronounced like the s in measure or the g in genre.

So teže sounds roughly like:

• TEH-zheh

Not exactly English, but that gets you close.

Also note:

• e in Croatian is usually a clear eh sound
• each letter is pronounced clearly

So teže is approximately TEH-zheh.

It can overlap with all of those, depending on context.

• In weather or air conditions, vlaga often means humidity
• In other contexts, it can mean moisture or dampness

In this sentence, because of u zraku (in the air), the natural meaning is clearly humidity.

So velika vlaga u zraku is best understood as high humidity in the air.