Ako nešto ne razumiješ, izravno pitaj učiteljicu.

A word-by-word breakdown is:

• Ako = if
• nešto = something
• ne razumiješ = you do not understand
• izravno = directly
• pitaj = ask (imperative: ask!)
• učiteljicu = the teacher / a teacher as the object of ask

So the structure is literally something like:

If something you do not understand, directly ask the teacher.

Natural English: If you don’t understand something, ask the teacher directly.

In Croatian, after ako (if), the present tense is very commonly used for real or possible situations:

• Ako dođe, reci mi. = If he comes, tell me.
• Ako imaš vremena, nazovi me. = If you have time, call me.

So:

• Ako nešto ne razumiješ = If you don’t understand something

This is normal Croatian grammar. English also often uses the present after if: If you don’t understand...

Pitaj is the imperative form of the verb pitati (to ask).

It is the 2nd person singular imperative, used when speaking to one person informally:

• pitaj! = ask!

So the sentence is giving an instruction or encouragement:

• izravno pitaj učiteljicu = ask the teacher directly

Other related forms:

• pitajte! = ask! (plural or formal singular)
• pitam = I ask
• pitaš = you ask
• pitati = to ask

Because učiteljicu is in the accusative case, which is used for the direct object of the verb pitati.

The base form is:

• učiteljica = teacher (nominative)

But after pitati (to ask), the person being asked is the direct object, so you use accusative:

• pitati učiteljicu = to ask the teacher

For feminine nouns ending in -a, the accusative singular usually changes to -u:

• učiteljica → učiteljicu
• sestra → sestru
• mama → mamu

No—nešto is correct here.

The sentence means:

• If there is something that you don’t understand...

Here, nešto does not mean the whole clause is negative. It means some particular thing may be unclear.

So:

• Ako nešto ne razumiješ = If you don’t understand something

You would not normally use ništa here, because ništa means nothing, which would change the meaning completely:

• Ako ništa ne razumiješ... = If you don’t understand anything...

That is a much stronger statement.

Yes, you can, but Ako nešto ne razumiješ sounds more natural and neutral in this context.

Both are understandable:

• Ako nešto ne razumiješ...
• Ako ne razumiješ nešto...

The first version is more common because the indefinite object nešto often appears before the verb in this kind of sentence.

So for a learner, the safest choice is:

• Ako nešto ne razumiješ...

Izravno means directly.

In this sentence, it suggests:

• ask the teacher yourself
• ask plainly, without hesitation
• go straight to the teacher

Yes, izravno and direktno are very close in meaning. In everyday speech, many people also say:

• direktno pitaj učiteljicu

But izravno is a very good standard Croatian choice.

Yes. Croatian word order is fairly flexible, although some positions sound more natural than others.

Possible versions include:

• Ako nešto ne razumiješ, izravno pitaj učiteljicu.
• Ako nešto ne razumiješ, pitaj učiteljicu izravno.
• Ako nešto ne razumiješ, pitaj izravno učiteljicu.

The first one sounds very natural and clear.

The second is also fine, but it can put a little more focus on izravno at the end.

The third is possible, but less natural for many speakers.

So the original sentence is a very good standard order.

It is informal singular.

You can tell because of:

• razumiješ = you understand (to one person, informal)
• pitaj = ask! (to one person, informal)

If you were speaking formally to one person, or to several people, you would use:

• Ako nešto ne razumijete, izravno pitajte učiteljicu.

So:

• razumiješ / pitaj = informal singular
• razumijete / pitajte = formal singular or plural

Because the sentence begins with a subordinate if-clause:

• Ako nešto ne razumiješ = dependent clause
• izravno pitaj učiteljicu = main clause

In Croatian, a clause introduced by ako is normally separated from the main clause by a comma when it comes first.

So:

• Ako pada kiša, ostajemo doma.
• Ako imaš pitanje, pitaj.
• Ako nešto ne razumiješ, izravno pitaj učiteljicu.

Razumiješ comes from the verb razumjeti = to understand.

Some common forms are:

• razumjeti = to understand
• razumijem = I understand
• razumiješ = you understand
• razumije = he/she understands

The -ije- spelling is standard in Croatian for this verb family.

You may also see related forms in other varieties of the language area, but for standard Croatian, razumiješ is exactly what you would expect.

Yes, absolutely.

• Ako nešto ne razumiješ, pitaj učiteljicu. = If you don’t understand something, ask the teacher.

Adding izravno gives a slightly stronger sense of ask directly / go straight to the teacher.

So:

• without izravno = simple instruction
• with izravno = slightly more emphasis on asking openly or going straight to the source

Both are correct.

Because Croatian usually does not need subject pronouns when the verb ending already shows who the subject is.

Here:

• razumiješ already means you understand
• pitaj already means ask! addressed to you

So ti (you) is unnecessary unless you want emphasis:

• Ako ti nešto ne razumiješ... would be odd here
• Ako nešto ne razumiješ, ti pitaj učiteljicu could emphasize you, but it is not the neutral version

The normal Croatian sentence leaves the pronoun out.