Kad dugo sjedim za računalom, vrat me počne boljeti i moram ustati.

Both kad and kada mean when.

• kad is the shorter, very common everyday form.
• kada is a bit fuller and can sound slightly more formal or emphatic.

In this sentence, kad is completely natural: Kad dugo sjedim za računalom, ...

You could also say: Kada dugo sjedim za računalom, ...

The meaning does not really change.

In Croatian, the present tense is often used for habitual or repeated actions, just as in English:

• Kad dugo sjedim za računalom... = When I sit at the computer for a long time...

Here sjedim does not mean only I am sitting right now. It can also mean whenever I sit or when I spend a long time sitting.

This is very normal in Croatian after kad.

Dugo means for a long time.

So:

• dugo sjedim = I sit for a long time
• kad dugo sjedim = when I sit for a long time

It is an adverb, so it modifies the verb sjedim.

A learner might expect something like longly, but Croatian simply uses dugo here.

Za računalom is the normal Croatian way to say at the computer in the sense of sitting and working/being positioned there.

• za + instrumental often means at / by / seated at
• za stolom = at the table
• za računalom = at the computer

So:

• sjediti za računalom = to sit at the computer

You may also hear na računalu, but that more often suggests on the computer in the sense of using the computer as a device/platform. In this sentence, the physical posture and location are important, so za računalom fits very well.

Because after za in this meaning, Croatian uses the instrumental case.

The base noun is:

• računalo = computer

Its instrumental singular form is:

• računalom

So:

• za računalom = at the computer

This is a useful pattern:

• za stolom = at the table
• za volanom = at the wheel / behind the wheel
• za računalom = at the computer

This is one of the most important things to notice.

Croatian often expresses pain with:

• the body part as the grammatical subject
• the person feeling the pain in the accusative pronoun

So:

• vrat me boli = my neck hurts
• literally: the neck hurts me

In your sentence:

• vrat = neck
• me = me
• počne boljeti = starts to hurt

So the structure is roughly: my neck starts hurting me

This is very idiomatic in Croatian.

Similar examples:

• Glava me boli. = My head hurts.
• Leđa me bole. = My back hurts.
• Ruka me boli. = My arm hurts.

Here vrat is in the nominative singular because it is the grammatical subject of počne boljeti.

So in:

vrat me počne boljeti

the subject is vrat = neck.

That may feel unusual to an English speaker, because English usually says my neck hurts, where my marks possession directly. Croatian often leaves the possessive unstated if it is obvious and uses the body part noun by itself.

So vrat me boli naturally means my neck hurts, not just a neck hurts me.

Because počne boljeti means starts to hurt, not simply hurts.

Compare:

• Vrat me boli. = My neck hurts.
• Vrat me počne boljeti. = My neck starts to hurt.

In your sentence, the idea is dynamic:

1. I sit at the computer for a long time.
2. Then my neck begins to hurt.
3. Then I have to get up.

So počne adds the meaning of the beginning of the pain.

After početi / počne (to begin / starts), Croatian normally uses an infinitive.

So:

• počne boljeti = starts to hurt
• počne raditi = starts working
• počne padati = starts falling

Here:

• počne is a present-tense form of početi
• boljeti is the infinitive to hurt / to ache

This is a very common verb pattern in Croatian.

Yes. Croatian word order is fairly flexible.

All of these are possible:

• Vrat me počne boljeti.
• Počne me boljeti vrat.

The second version is also very common and may sound a bit more natural to some speakers in conversation.

The difference is mostly one of focus and rhythm, not core meaning.

Your original sentence is absolutely correct.

Because Croatian often does not use possessive words like my with body parts when the owner is already clear from context.

So instead of saying:

• moj vrat me boli

Croatian usually prefers:

• vrat me boli

The pronoun me already tells us whose neck is meant: mine.

Using moj is possible, but it is usually emphatic, contrastive, or unnatural if there is no special reason to stress it.

Because ustati is the perfective verb meaning to stand up / get up once, and that fits this sentence.

• moram ustati = I have to stand up / get up
• ustajati would suggest repeated or habitual standing up, which is not the point here

The sentence describes a single necessary action that follows the neck pain: I have to get up.

So the perfective infinitive ustati is the natural choice.

It can mean either, depending on context.

In this sentence, because the speaker has been sitting at the computer, ustati most naturally means:

• to stand up
• or more loosely to get up from the chair

So: moram ustati = I have to stand up

Because Kad dugo sjedim za računalom is a subordinate clause introduced by kad (when), and Croatian normally separates that kind of clause with a comma.

So the structure is:

• Kad dugo sjedim za računalom, = subordinate clause
• vrat me počne boljeti i moram ustati. = main clause

This comma is standard Croatian punctuation.

Yes, računalo is a standard Croatian word for computer.

You may also hear:

• kompjuter or kompjutor in informal speech, depending on region and speaker

But računalo is a very good, neutral, standard choice, especially in written Croatian.

So za računalom is completely natural and standard.

Yes. Very naturally.

Because of kad + present tense, the sentence often has a general, repeated meaning:

• Whenever I sit at the computer for a long time, my neck starts to hurt and I have to get up.

It can also fit a specific situation depending on context, but the most natural reading is habitual or general.