Kad je zrak hladan, držim usta zatvorena.

Both kad and kada mean when.

• kad is shorter and very common in everyday speech
• kada is slightly more formal or a bit more emphatic

So both of these are fine:

• Kad je zrak hladan, držim usta zatvorena.
• Kada je zrak hladan, držim usta zatvorena.

In normal conversation, kad is extremely natural.

Because je is a clitic form of biti (to be), and Croatian clitics usually go in the second position in their clause.

So the natural order is:

• Kad je zrak hladan

not:

• Kad zrak je hladan

This is a very common Croatian pattern. English speakers often expect the verb to stay after the subject, but Croatian clitics follow different placement rules.

Croatian has no articles, so there is no direct equivalent of the or a/an.

So:

• zrak can mean air, the air, or sometimes an air depending on context

In this sentence, English naturally translates it as the air, but Croatian simply says zrak.

Because hladan agrees with zrak.

• zrak is masculine singular
• the adjective must match it in gender, number, and case

So:

• hladan zrak = cold air
• zrak je hladan = the air is cold

If you said hladno, that would usually be used differently, for example in an impersonal sentence:

• Hladno je. = It is cold.

So here, because the adjective describes the noun zrak, it must be hladan.

Zrak is in the nominative singular because it is the subject of the clause:

• Kad je zrak hladan = When the air is cold

The subject of a sentence is normally in the nominative, and here zrak is the thing that is cold.

Držim is the 1st person singular of držati, which often means:

• to hold
• to keep

In this sentence, it means something like I keep:

• držim usta zatvorena = I keep my mouth closed

This is a very natural Croatian structure:

• držati + object + adjective/participle

For example:

• Držim vrata otvorena. = I keep the door open.
• Držim prozor zatvoren. = I keep the window closed.

So here it is not literally about holding your mouth with your hand; it means maintaining it in that state.

Because usta is a noun that is normally used in the plural form in Croatian, even when it refers to one person’s mouth.

So:

• usta = mouth

This is one of those places where Croatian and English organize meaning differently. English uses a singular noun, but Croatian uses a plural-form noun.

That is why the words agreeing with usta are also plural:

• usta zatvorena

Even though the meaning in English is singular, the Croatian grammar treats usta as plural.

Here usta is in the accusative, because it is the direct object of držim:

• držim što? → usta

However, for this noun, the accusative form looks the same as the nominative:

• nominative: usta
• accusative: usta

So even though the form does not change, its role in the sentence is accusative.

Zatvorena agrees with usta and describes the state they are kept in.

You can think of it as:

• držim usta zatvorena = I keep my mouth closed

Grammar-wise:

• usta is treated as neuter plural
• so the matching form is zatvorena

That is why it is not:

• zatvoren (masculine singular)
• zatvorenu (feminine singular accusative)
• zatvorene (not the correct neuter plural form here)

This pattern is very common in Croatian:

• držati nešto otvoreno/zatvoreno/spremno
  = to keep something open/closed/ready

Because usta is neuter plural, not masculine or feminine plural.

For many adjectives, the nominative/accusative plural endings are:

• masculine animate: different pattern
• masculine inanimate / feminine plural often: -i / -e depending on form
• neuter plural: -a

So:

• zatvorena usta = correct

Since usta is neuter plural, zatvorena is the form that matches.

Croatian often leaves out possessive words like my, your, his, especially with body parts, when the owner is obvious from the verb.

So:

• držim usta zatvorena naturally means I keep my mouth closed

Because the verb is držim (I keep), it is already clear whose mouth is meant.

You could add a possessive for emphasis, but it would usually sound unnecessary here.

Yes. Croatian word order is fairly flexible, as long as the clitic rules are respected.

For example, you could also say:

• Držim usta zatvorena kad je zrak hladan.

That means the same thing: I keep my mouth closed when the air is cold.

The difference is mostly about emphasis and flow:

• Kad je zrak hladan, držim usta zatvorena.
  starts with the condition/time frame
• Držim usta zatvorena kad je zrak hladan.
  starts with the main action

But inside the clause, je still needs to stay in its normal clitic position:

• kad je zrak hladan is natural
• kad zrak je hladan is not

Yes, and it means something slightly broader:

• Kad je zrak hladan = When the air is cold
• Kad je hladno = When it is cold

The original sentence specifically mentions the air. The version with hladno is more general and may sound a bit more natural in some everyday situations, depending on what exactly you want to emphasize.