Kurir radi ceo dan.

• Kurir = a masculine singular noun, the subject of the sentence
• radi = the verb, 3rd person singular present tense of raditi
• ceo dan = a time expression meaning all day

So the structure is basically:

subject + verb + duration of time

Because the subject kurir is third person singular: he/she/the courier.

The present tense of raditi goes like this:

• ja radim = I work
• ti radiš = you work
• on/ona radi = he/she works
• mi radimo = we work
• vi radite = you work
• oni rade = they work

So with kurir, you need radi.

It can mean both.

Serbian present tense often covers both:

• works (habitual / general meaning)
• is working (right now / in progress)

So Kurir radi ceo dan could mean:

• The courier works all day
  or
• The courier is working all day

The exact nuance depends on context.

Because Serbian does not have articles like English a / an / the.

So kurir can mean:

• a courier
• the courier

Context tells you which one is meant.

If Serbian wants to be more specific, it can use other words, such as:

• taj kurir = that courier
• ovaj kurir = this courier

Ceo means whole / entire / all, and it agrees with dan.

Here, ceo dan means the whole day / all day.

A useful point:

• dan is masculine singular
• the expression for duration of time uses the accusative
• but for masculine inanimate nouns, the accusative singular looks the same as the nominative singular

So:

• nominative: ceo dan
• accusative: ceo dan

That is why the form stays ceo dan.

In Serbian, the accusative is used not only for direct objects, but also for some time expressions, especially duration.

So ceo dan literally means something like for the whole day / all day.

This is very common:

• ceo dan = all day
• celu noć = all night
• ceo mesec = all month

So this is a normal accusative-of-duration pattern.

Yes, it is a little unusual.

The masculine singular form is ceo, but other forms use cel-:

• ceo dan = whole day
• cela noć = whole night
• celo jutro = whole morning
• celog dana = of the whole day

So learners often notice that ceo does not look exactly like the other forms. That is normal.

Yes, Serbian word order is fairly flexible.

Kurir radi ceo dan is the most neutral and straightforward order.

Other orders are possible, but they change emphasis:

• Ceo dan kurir radi = emphasis on all day
• Radi kurir ceo dan = marked, more poetic or contrastive in most contexts

For a beginner, Kurir radi ceo dan is the safest neutral version.

Raditi is the normal imperfective verb for to work.

With a person as subject, it often means:

• to work
• to be at work
• to do one’s job

In other contexts, raditi can also mean things like:

• to function / operate
  
  • Mašina radi. = The machine is working.
• to do
  
  • depending on the sentence

But in Kurir radi ceo dan, the meaning is clearly works / is working.

Because raditi and uraditi are not the same kind of verb.

• raditi = imperfective, ongoing or repeated activity
• uraditi = perfective, to do something to completion

Here the sentence describes an activity lasting through the day, so raditi is the right choice.

Compare:

• Kurir radi ceo dan. = The courier works all day.
• Kurir je uradio posao. = The courier finished the job.

So uraditi would suggest completing something, not simply working throughout the day.

A beginner-friendly approximation is TSEH-o.

Two helpful points:

• Serbian c is pronounced like ts
• ceo is not pronounced like English see-oh

So:

• ceo ≈ tseh-o

Also:

• kurir ≈ koo-reer
• radi ≈ rah-dee

Those are only rough English-based approximations, but they are good enough to get started.

No, but it is one of the most common and neutral ways.

Other possibilities include:

• čitav dan = the whole day
• po ceo dan = all day long / for the whole day, depending on context

Still, ceo dan is very natural and probably the first version a learner should know.