Learn Hardcore Croatian: Kad nemam dovoljno pažnje, moram ponoviti zadatak još jednom. - When I do not have enough attention, I have to repeat the exercise once more.

Kad and kada have the same meaning. Kada is slightly more formal / careful, and kad is a very common everyday shortened form. In most contexts you can freely switch them:

Kada nemam dovoljno pažnje, moram ponoviti zadatak još jednom.
Kad nemam dovoljno pažnje, moram ponoviti zadatak još jednom.

Both are correct.

Why is it nemam and not ne imam?

The negative ne is written together with imati (to have) in the first person singular:

imam = I have
nemam = I don’t have (not ne imam)

So nemam is one word in standard spelling. The same happens with other forms of imati:

ima → nema (he/she/it has → doesn’t have)
imaju → nemaju (they have → don’t have)

Why is it dovoljno pažnje and not dovoljno pažnja?

Because dovoljno (enough) is used like a quantity word, and in Croatian, many quantity words take the genitive case.

pažnja = attention (nominative)
pažnje = of attention (genitive singular)

After words like dovoljno, puno, mnogo, malo, etc., the noun normally goes to genitive:

dovoljno pažnje – enough attention
puno vremena – a lot of time
malo strpljenja – a little patience

So nemam dovoljno pažnje literally is “I don’t have enough of attention”.

Can you explain the word pažnje more? What is its base form and meaning?

The base form (dictionary form) is pažnja (feminine noun):

pažnja = attention, care, concentration
pažnje = genitive singular (of attention)

Some common uses:

Obrati pažnju. – Pay attention.
Hvala na pažnji. – Thank you for your attention.
Nedostaje mi pažnje. – I lack attention. / I don’t have enough focus.

In the sentence nemam dovoljno pažnje, the form pažnje is required by dovoljno (see the previous answer).

Why is there a comma before moram? How does punctuation work with kad (“when”) clauses?

When a subordinate clause with kad/kada comes before the main clause, Croatian normally uses a comma:

Kad nemam dovoljno pažnje, moram ponoviti zadatak još jednom.
When I don’t have enough attention, I have to repeat the task once again.

If the kad clause comes after the main clause, a comma is usually not written:

Moram ponoviti zadatak još jednom kad nemam dovoljno pažnje.
(Stylistically a bit odd here, but punctuated correctly.)

So the rule of thumb:

Subordinate clause first → comma between clauses.
Main clause first → usually no comma.

What does moram mean exactly, and how is it different from trebam?

Moram is 1st person singular of morati = must, have to:

Moram ponoviti zadatak. – I must repeat the task. / I have to repeat the task.

Trebam is 1st person singular of trebati. It usually means “I need”, not “I must”:

Trebam ponoviti zadatak. – I need to repeat the task. (It would be good / necessary for me.)
Moram ponoviti zadatak. – I must repeat the task. (Stronger obligation, maybe from rules, teacher, boss, etc.)

In casual speech people sometimes blur this difference, but moram is the more precise choice for obligation (“have to / must”) in this sentence.

Why is the verb ponoviti (perfective) used with moram? Could it be ponavljati?

Croatian distinguishes aspect:

ponoviti = perfective, “to repeat once (as a single completed action)”
ponavljati = imperfective, “to repeat (habitually, continuously, over time)”

With moram in this context, you want to express one whole, complete repetition of the task:

Moram ponoviti zadatak još jednom.
= I have to repeat the task once more (do it again, one more whole time).

If you said:

Moram ponavljati zadatak.

that would suggest “I have to keep repeating the task” / “I have to do the task repeatedly/over and over,” which has a different nuance (repeated or ongoing activity, not one extra repetition).

Why is it zadatak and not some changed form? Which case is it?

Zadatak is a masculine noun meaning task / assignment / exercise.

In the sentence, zadatak is the direct object of ponoviti (“repeat what?” → the task), so it is in the accusative singular.

For most masculine inanimate nouns, the accusative singular is identical to the nominative:

Nominative: zadatak – the task (subject)
Accusative: ponoviti zadatak – to repeat the task (object)

So the form zadatak here is accusative, even though it looks like nominative.

What does još jednom literally mean, and how is it different from opet or ponovno?

Još jednom literally means “one more time” / “once more / once again”:

još = still, more, additional
jednom = once (adverb; historically from “one time”)

So:

još jednom – one more time, once more

Other options:

opet – again (neutral, very common)
- Moram opet ponoviti zadatak. – I have to repeat the task again.
ponovno – again, anew (a bit more “formal” or careful than opet)
- Moram ponovno ponoviti zadatak. – I have to repeat the task again.

In many contexts you can interchange them, but:

još jednom explicitly emphasizes “one more time” (an extra repetition, often counted).
opet / ponovno = just “again,” with less explicit focus on one additional time.

Can the word order be changed? For example, can I say Moram zadatak ponoviti još jednom?

Yes, Croatian word order is relatively flexible. All of these are grammatically correct, with small differences in emphasis:

Moram ponoviti zadatak još jednom.
Moram još jednom ponoviti zadatak.
Moram zadatak ponoviti još jednom.

They all mean essentially: “I have to repeat the task once more.”

Putting još jednom closer to ponoviti can slightly emphasize the “one more time” aspect of the action.
Moving zadatak earlier can put a bit more focus on the task itself.

For a learner, version 1 or 2 is the safest and most natural-sounding.

Where is the word for “I” in this sentence? Why isn’t ja used?

The subject “I” is built into the verb form in Croatian. The ending -am in nemam and moram already shows that the subject is 1st person singular:

(Ja) nemam dovoljno pažnje. – I don’t have enough attention.
(Ja) moram ponoviti zadatak još jednom. – I have to repeat the task once more.

The pronoun ja is usually omitted unless you want to emphasize it:

Ja moram ponoviti zadatak, a ti ne moraš.
I have to repeat the task, but you don’t.

So the full sentence with an explicit pronoun would be:

Kad ja nemam dovoljno pažnje, moram ponoviti zadatak još jednom.

…but normally Croatians just drop ja here.

Could you say this idea in another common way in Croatian?

Yes, there are several natural variants with very similar meaning. For example:

Kad nisam dovoljno pažljiv, moram ponoviti zadatak još jednom.
When I am not careful/attentive enough, I have to repeat the task once more.
(Uses the adjective pažljiv – attentive, careful.)
Kad se ne koncentriram dovoljno, moram ponoviti zadatak još jednom.
When I don’t concentrate enough, I have to repeat the task once more.

Your original sentence is perfectly correct; these are just alternative ways Croatians might also express the same idea.