Udahni duboko prije nego što počneš govoriti.

Udahni is the imperative form, used to give a command or instruction to one person in an informal way.

• infinitive: udahnuti = to inhale / to breathe in
• imperative (2nd person singular): udahni = inhale!

So Udahni duboko means Take a deep breath or more literally Inhale deeply.

Because duboko is an adverb, not an adjective.

• dubok / duboka / duboko as an adjective = deep
• duboko as an adverb = deeply

Here it describes how you should inhale, so Croatian uses the adverb:

• Udahni duboko. = Breathe in deeply.

This is very similar to English deep vs deeply, although English often says take a deep breath instead of breathe deeply.

Prije nego što means before when it introduces a whole clause.

So:

• prije = before
• nego što helps link it to another action or event

In this sentence:

• prije nego što počneš govoriti = before you start speaking

A useful pattern is:

• prije nego što + clause

For example:

• Prije nego što izađeš, zatvori prozor.
• Before you go out, close the window.

Not really. In this sentence, što is not being used with its usual standalone meaning what. It is part of the fixed linking expression prije nego što.

So you should learn prije nego što as a whole connector meaning:

• before
• more literally, before ... happens / before ... you do ...

You do not need to translate što separately here.

This is mainly about aspect.

• početi = to start, to begin → perfective
• počinjati / počinjati forms are imperfective and focus more on an ongoing or repeated beginning

In Croatian, after time expressions like prije nego što, a perfective present form is often used for a future event:

• prije nego što počneš govoriti = before you start speaking

So even though počneš is formally a present-tense form, here it refers to something that will happen next.

This is very normal in Croatian.

Because after početi (to start), Croatian usually uses an infinitive:

• početi govoriti = to start speaking

So the structure is:

• počneš govoriti = you start speaking

If you said počneš govoriš, that would be incorrect.

A very common pattern is:

• početi + infinitive
• nastaviti + infinitive
• prestati + infinitive

Examples:

• Počeo je raditi. = He started working.
• Prestani pričati. = Stop talking.

Yes, you may hear or see prije nego počneš govoriti, and it is understandable. But prije nego što is a very common and standard full form.

So:

• prije nego što počneš govoriti = fully standard and very common
• prije nego počneš govoriti = also possible in many contexts

For a learner, it is safest to learn and use prije nego što first.

It is informal singular.

You can tell because of:

• udahni = command to one person, informal
• počneš = you start, singular informal

If you wanted to say it formally to one person, or to more than one person, you would use:

• Udahnite duboko prije nego što počnete govoriti.

That can mean:

• speaking politely to one person, or
• speaking to several people

Yes. Croatian word order is fairly flexible.

The original sentence:

• Udahni duboko prije nego što počneš govoriti.

A very natural alternative is:

• Prije nego što počneš govoriti, udahni duboko.

Both mean the same thing. The difference is mostly about focus:

• starting with Udahni duboko emphasizes the instruction first
• starting with Prije nego što... emphasizes the time relationship first

Both are good Croatian.

Because govoriti means to speak / to be speaking, while reći means to say / to tell a specific thing.

In this sentence, the idea is:

• before you start speaking

That is general, so govoriti fits well.

If you used reći, the meaning would shift more toward before you say something.

Compare:

• prije nego što počneš govoriti = before you start speaking
• prije nego što nešto kažeš = before you say something

Yes, it is very natural. It is exactly the kind of thing you might hear:

• from a doctor
• from a speech coach
• from someone giving advice
• in relaxation or breathing instructions

It sounds clear, idiomatic, and normal.

A very common formal version is:

• Udahnite duboko. = Take a deep breath.

So the whole sentence sounds completely natural in Croatian.