U zadatku piše da treba ispraviti zamjenicu i podcrtati veznik.

Because the preposition u takes the locative case when it means in or inside a place.

• zadatak = nominative singular
• u zadatku = locative singular, meaning in the exercise

If u expressed movement into something, then Croatian would use the accusative:

• Staviti nešto u zadatak = to put something into the exercise

But here the meaning is location, so locative is required.

In this sentence, piše is used impersonally. It does not mean that some specific person is writing right now. Instead, it means something like:

• it says
• it is written
• the instructions say

So U zadatku piše... is a very common Croatian way to say In the exercise, it says...

There is no need to name a subject such as the book or the exercise. Croatian often leaves this kind of subject unexpressed when the meaning is obvious.

Croatian often uses da to introduce a clause after verbs like piše, kaže, misli, zna, and so on.

So:

• piše da... = it says that...

Then inside that clause, treba means it is necessary / one should / one needs to.

So the structure is:

• U zadatku piše = In the exercise it says
• da treba ispraviti... = that one should correct...

This is completely natural Croatian. English may prefer it says to correct... or it says that you should correct..., but Croatian commonly uses da + clause here.

Treba here is also impersonal. It means:

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

So treba ispraviti zamjenicu means the pronoun should be corrected or you should correct the pronoun.

There is no explicit you, but in instructions that is often understood automatically. Croatian frequently uses this impersonal style for directions and school tasks.

They are infinitives because they depend on treba.

• treba ispraviti = should correct
• treba podcrtati = should underline

This is similar to English should correct and should underline, where the second verb stays in its base form.

So the pattern is:

• treba + infinitive

Very common examples:

• Treba učiti. = One should study.
• Treba napisati odgovor. = One should write the answer.

Both zamjenicu and veznik are direct objects, so they must be in the accusative case.

• zamjenica is a feminine noun

  • nominative: zamjenica
  • accusative singular: zamjenicu

• veznik is a masculine inanimate noun

  • nominative singular: veznik
  • accusative singular: veznik
    For masculine inanimate nouns, accusative often looks the same as nominative.

So:

• ispraviti zamjenicu = correct the pronoun
• podcrtati veznik = underline the conjunction

Because both infinitives are coordinated by i and both depend on the same treba.

So this part means:

• treba ispraviti zamjenicu i podcrtati veznik
• one should correct the pronoun and underline the conjunction

Croatian does not need to repeat treba here, just as English usually does not repeat should:

• You should correct the pronoun and underline the conjunction.

You could repeat it for emphasis, but it is unnecessary:

• ...da treba ispraviti zamjenicu i treba podcrtati veznik
  This sounds heavier and less natural in a simple instruction.

I is the conjunction meaning and.

It connects the two infinitive phrases:

• ispraviti zamjenicu
• podcrtati veznik

Since the school instruction says to underline the conjunction, this is probably the word they want the student to underline: i.

So grammatically, i is a coordinating conjunction.

They are perfective because the sentence refers to single completed actions in an exercise.

• ispraviti = to correct, to make right completely
• podcrtati = to underline, as a completed act

In task instructions, Croatian often prefers perfective verbs because the goal is to finish the action once.

Compare:

• ispraviti zamjenicu = correct the pronoun

• ispravljati zamjenicu = be correcting / correct repeatedly / be in the process of correcting

• podcrtati veznik = underline the conjunction
• podcrtavati veznik = keep underlining / underline repeatedly

For classroom instructions, the perfective forms are the normal choice.

Yes, Croatian word order is fairly flexible, though some versions sound more natural than others.

Original:

• U zadatku piše da treba ispraviti zamjenicu i podcrtati veznik.

Possible alternatives:

• Piše u zadatku da treba ispraviti zamjenicu i podcrtati veznik.
• U zadatku piše da zamjenicu treba ispraviti i podcrtati veznik.

But the original sounds very natural and clear. Putting U zadatku first sets the context right away: In the exercise...

So while Croatian allows flexibility, not all word orders are equally neutral or elegant.

Croatian has no articles like English a/an/the.

So nouns like:

• zadatak
• zamjenica
• veznik

can mean:

• an exercise / the exercise
• a pronoun / the pronoun
• a conjunction / the conjunction

The exact meaning is understood from context. In this sentence, the context clearly makes them definite:

• the exercise
• the pronoun
• the conjunction

That is why Croatian can express the idea without separate words for the.