Ako nešto nije jasno, profesorica kaže da je bolje odmah pitati nego poslije morati ispraviti cijeli zadatak.

Jasno is the neuter singular short-form adjective/adverbial predicate used in impersonal expressions like:

• To je jasno. = That is clear.
• Nije jasno. = It is not clear.

In Ako nešto nije jasno, the word nešto means something, and Croatian naturally uses jasno in this neutral predicate style. So the idea is:

• ako nešto nije jasno = if something is not clear

This is a very common Croatian pattern.

Nešto is in the nominative/accusative singular neuter form. Here it is the subject-like element of nije jasno:

• nešto nije jasno = something is not clear

Because nešto is an indefinite pronoun meaning something, its form does not change here from what you usually see in dictionary form.

Because Ako nešto nije jasno is a subordinate if-clause. In Croatian, clauses introduced by ako are normally separated from the main clause by a comma.

So the structure is:

• Ako nešto nije jasno, = If something is not clear,
• profesorica kaže... = the professor says...

This is standard punctuation.

Da introduces a subordinate clause, often equivalent to English that.

So:

• profesorica kaže = the professor says
• da je bolje... = that it is better...

Croatian uses da very often after verbs of saying, thinking, knowing, etc.

Because bolje here is part of the predicate je bolje = it is better.

So:

• da je bolje odmah pitati... = that it is better to ask immediately...

Croatian often uses this impersonal structure:

• Bolje je čekati. = It is better to wait.
• Bolje je pitati. = It is better to ask.

In your sentence, that becomes part of a subordinate clause after da:

• kaže da je bolje...

Because bolje here does not describe a noun like better professor or better book. It is being used impersonally, meaning it is better.

Compare:

• bolja knjiga = a better book
• bolji student = a better student
• bolje pitati / bolje je pitati = it is better to ask

So bolje here functions like a general comparative expression, not like an adjective agreeing with a noun.

Because after bolje (je), Croatian commonly uses an infinitive to express a general action:

• Bolje je pitati. = It is better to ask.
• Bolje je pričekati. = It is better to wait.

So in your sentence:

• da je bolje odmah pitati = that it is better to ask right away

This is a very normal Croatian structure.

Nego means than in comparisons.

Here the comparison is between two actions:

• bolje odmah pitati
• nego poslije morati ispraviti cijeli zadatak

So the meaning is:

• it is better to ask immediately than to have to correct the whole assignment later

Croatian often uses:

• bolje ... nego ... = better ... than ...

Because morati is a modal verb meaning must / have to, and modal verbs are followed by another infinitive.

So:

• morati = to have to
• ispraviti = to correct

Together:

• morati ispraviti = to have to correct

Since this whole part depends on bolje ... nego ..., it stays in the infinitive phrase:

• bolje odmah pitati nego poslije morati ispraviti...

Ispraviti is perfective, meaning to correct something as a completed action.
Ispravljati is imperfective, meaning correcting in progress or repeatedly.

Here the idea is a single completed result:

• ispraviti cijeli zadatak = to correct the whole assignment

That is why ispraviti fits well.

Odmah means immediately / right away.

So:

• odmah pitati = to ask immediately

Its position before the infinitive is very natural and emphasizes the timing of the action. Croatian word order is flexible, but this placement is the most neutral here.

Poslije means later / afterwards here.

In many contexts, poslije and kasnije are quite similar:

• poslije = later, afterwards
• kasnije = later

In this sentence, poslije works very naturally:

• nego poslije morati ispraviti... = than later having to correct...

It contrasts nicely with odmah:

• odmah = immediately
• poslije = later

Cijeli zadatak is in the accusative singular masculine animate/inanimate form, which here looks the same as the nominative because zadatak is inanimate masculine.

It is the direct object of ispraviti:

• ispraviti što? → cijeli zadatak
• correct what? → the whole assignment

Also, cijeli agrees with zadatak in gender, number, and case:

• masculine singular accusative inanimate: cijeli zadatak

Yes, that would also be grammatical, but it would mean something slightly different.

Your sentence says:

• bolje odmah pitati nego poslije morati ispraviti cijeli zadatak
• better to ask immediately than later have to correct the whole assignment

This emphasizes the unpleasant necessity: having to correct it later.

If you say:

• bolje je odmah pitati nego kasnije ispravljati cijeli zadatak

that focuses more on the activity of correcting later, without explicitly saying have to.

So the original sentence sounds a bit stronger and more practical.

It is natural and completely normal Croatian. It sounds like something a teacher really might say.

It has a mildly careful, standard tone because of the full structure:

• Ako nešto nije jasno, profesorica kaže da je bolje odmah pitati nego poslije morati ispraviti cijeli zadatak.

In everyday speech, someone might shorten it a bit, but the original is fully idiomatic and standard.