Doktorica kaže da treba držati usta zatvorena kad je zrak hladan i vjetar jak.

Doktorica is the feminine form of doktor.

• doktor = a male doctor / doctor in the masculine form
• doktorica = a female doctor

So this sentence tells you the speaker is referring to a woman doctor.

Kaže da means says that.

In Croatian, after verbs like reći / kazati and misliti, it is very common to use da plus a finite verb:

• Kaže da dolazi. = She says that she is coming.
• Kaže da treba... = She says that it is necessary / one should...

So da introduces the content of what the doctor says.

Here treba is being used impersonally.

That means it does not refer to a specific person like I must or you must. Instead, it means something like:

• it is necessary to
• one should
• you should (in a general sense)

So treba držati usta zatvorena means roughly one should keep one’s mouth closed.

This is a very common Croatian structure for giving general advice or instructions.

Because after impersonal treba, Croatian commonly uses the infinitive to name the action that is necessary.

So:

• treba držati = one should keep
• treba piti = one should drink
• treba čekati = one should wait

This is a very standard pattern:

• treba + infinitive

This is one of the most important vocabulary points in the sentence.

Usta is normally used in the plural form, even when it refers to one person’s mouth. This is similar to how some languages use a plural form for body parts that English treats as singular.

So in Croatian:

• usta = mouth
• literally it looks plural, but it is the normal word

Examples:

• Otvori usta. = Open your mouth.
• Drži usta zatvorena. = Keep your mouth closed.

So even though English says mouth, Croatian uses usta.

Because zatvorena agrees with usta.

Since usta behaves as a neuter plural noun, the adjective describing it must also be in the neuter plural form:

• usta = neuter plural
• zatvorena = neuter plural

So:

• držati usta zatvorena = to keep the mouth closed

This adjective is not agreeing with doktorica or with any hidden person. It agrees specifically with usta.

Usta is in the accusative because it is the direct object of držati.

• držati što? = to keep what?
• usta

With this noun, the nominative and accusative forms look the same:

• nominative: usta
• accusative: usta

So the form does not change, but the function here is accusative.

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

So:

• zrak can mean air or the air
• vjetar can mean wind or the wind

The exact meaning comes from context. In this sentence, English naturally uses the air and the wind, but Croatian does not need separate words for that.

Kad and kada both mean when.

• kada is the full form
• kad is the shorter, very common everyday form

In this sentence, kad is completely natural. You could also say kada je zrak hladan i vjetar jak, and the meaning would be the same.

Croatian often leaves out a repeated to be verb when two similar parts are joined.

So this part means:

• kad je zrak hladan i vjetar jak
• literally: when the air is cold and the wind strong
• naturally: when the air is cold and the wind is strong

The second je is understood. Croatian often does this to avoid repetition.

You could also say:

• kad je zrak hladan i vjetar je jak

But the version in your sentence is very natural and concise.

They agree with the nouns they describe:

• zrak is masculine singular, so: hladan
• vjetar is masculine singular, so: jak

So:

• zrak hladan = air cold
• vjetar jak = wind strong

In full predicate structure:

• zrak je hladan
• vjetar je jak

Both adjectives are masculine singular nominative because both nouns are masculine singular subjects.

Yes, it is a very common and natural expression.

It literally means to keep the mouth closed, but depending on context it can mean:

• physically keep your mouth closed
• do not breathe through your mouth
• sometimes even keep quiet in other contexts

In this sentence, because of the mention of cold air and strong wind, it clearly means the physical sense: keep your mouth closed so you breathe in a better way, usually through the nose.

Yes, but the tone would change a little.

• treba držati... = one should keep... / it is advisable to...
• trebate držati... = you should keep... (directly addressing someone)
• mora se držati... = it must be kept... / one must keep... (stronger, more forceful)

The sentence with treba sounds like general medical advice, which fits very well here.