Ako nešto ne razumiješ, učiteljica će ti to dodatno objasniti.

Ako means if.

The first part, Ako nešto ne razumiješ, is an if-clause. The second part, učiteljica će ti to dodatno objasniti, is the main clause.

Croatian normally uses a comma after an introductory if-clause, just as English often does:

Ako nešto ne razumiješ, učiteljica će ti to dodatno objasniti.

So the comma separates the condition from the result.

This is very normal in Croatian.

After ako for a real future condition, Croatian often uses the present tense, not the future:

• Ako nešto ne razumiješ... = If you do not understand something...
• then the result is in the future: ...učiteljica će ti to objasniti.

English does something very similar:

• If you don’t understand something, the teacher will explain it.

So Croatian is not saying you already do not understand it right now only; it can also refer to a possible future situation.

Because the verb ending already shows the subject.

Razumiješ means you understand. The ending -š tells you it is 2nd person singular.

So Croatian often leaves out the subject pronoun ti when it is already clear from the verb:

• razumiješ = you understand
• ne razumiješ = you do not understand

You could add ti for emphasis, but it is not necessary:

• Ako ti nešto ne razumiješ... would sound marked or contrastive, not neutral.

Because nešto means something, while ništa means nothing.

So:

• Ako nešto ne razumiješ = If there is something you do not understand
• Ako ništa ne razumiješ = If you understand nothing

Those are different meanings. In this sentence, the idea is that maybe one part is unclear, not that everything is unclear.

In Croatian, ne is usually written as a separate word before the verb:

• ne razumiješ = you do not understand
• ne znam = I do not know
• ne mogu = I cannot

So ne razumiješ is the standard way to negate the verb.

Učiteljica means female teacher. It is the feminine noun.

• učitelj = male teacher
• učiteljica = female teacher

In this sentence, učiteljica is the subject, so it is in the nominative case.

A native speaker would understand that the teacher being referred to is female.

Croatian commonly forms the future with:

će + infinitive

Here:

• će = future auxiliary
• objasniti = infinitive, to explain

So:

• učiteljica će objasniti = the teacher will explain

This is the normal future I form.

Also notice that će usually comes very early in the clause, often in second position:

• Učiteljica će ti to dodatno objasniti.

Ti here means to you.

It is the dative form of the pronoun ti:

• nominative: ti = you
• dative: ti = to you

So in this sentence:

• učiteljica će ti to objasniti
• literally: the teacher will explain that to you

It is the indirect object.

To means that or it, referring to the thing you did not understand.

So the structure is:

• nešto = something
• later, to = that / it, meaning that same unclear thing

This is very natural in Croatian. English does the same kind of thing:

• If you don’t understand something, the teacher will explain it.

Here, to is the direct object of objasniti.

Dodatno means something like:

• additionally
• further
• in more detail
• extra

It is an adverb, because it describes how the teacher will explain.

So:

• objasniti = explain
• dodatno objasniti = explain further / explain in more detail

In this sentence, it adds the idea that the teacher will give extra explanation, not just a basic one.

Croatian word order is fairly flexible, but this version is neutral and natural.

A few things are happening:

• učiteljica comes first as the subject
• će comes early because it is an unstressed auxiliary
• ti also tends to come early
• to and dodatno come before the infinitive objasniti

So the sentence flows naturally as:

Učiteljica će ti to dodatno objasniti.

Some word order changes are possible, but they usually change emphasis:

• To će ti učiteljica dodatno objasniti.
  Emphasis on that
• Učiteljica će to dodatno objasniti ti.
  Not natural in standard neutral usage

So yes, word order can change, but the original is a very standard choice.

This is about aspect, which is very important in Croatian verbs.

• objasniti is perfective
• objašnjavati is imperfective

Objasniti focuses on a completed result: the teacher will explain it successfully, as a whole.

That fits this sentence well:

• učiteljica će ti to objasniti = the teacher will explain it to you

If you used objašnjavati, it would suggest more of an ongoing or repeated process:

• učiteljica će ti to objašnjavati = the teacher will be explaining it to you / will explain it repeatedly

So objasniti is the natural choice when you mean one complete explanation.

Yes. Razumiješ is standard Croatian spelling.

Croatian standard language normally uses ije/je in words like this, so you get:

• razumijem
• razumiješ
• razumije

This is exactly what you would expect in standard Croatian.