Sve više razumijem hrvatski kad učiteljica govori polako.

Sve više means more and more.

It is a very common Croatian expression used to show gradual increase:

• Sve više razumijem = I understand more and more
• Sve više učim = I am learning more and more

Literally, it does not translate word-for-word in a natural English way, so it is best to learn it as a fixed phrase.

Razumijem is the 1st person singular present tense of razumjeti = to understand.

So:

• ja razumijem = I understand

In this sentence, the subject ja is not stated because Croatian usually leaves subject pronouns out when the verb already makes the person clear.

So:

• Razumijem hrvatski = I understand Croatian

Yes, the full expression would be hrvatski jezik = the Croatian language.

But in Croatian, just like in English, the language name is often used by itself:

• učim hrvatski = I’m learning Croatian
• govorim hrvatski = I speak Croatian
• razumijem hrvatski = I understand Croatian

So hrvatski here means the Croatian language, even without jezik.

It is in the accusative case because it is the direct object of razumijem.

You understand what?
→ hrvatski

Since hrvatski here stands for hrvatski jezik, and jezik is masculine inanimate, the accusative form is the same as the nominative:

• nominative: hrvatski
• accusative: hrvatski

So the form does not change, even though the case is accusative.

Kad and kada both mean when.

In this sentence, they are interchangeable:

• kad učiteljica govori polako
• kada učiteljica govori polako

The shorter form kad is very common in everyday speech and writing. Kada can sound a little more formal or more explicit, but both are correct.

Because after kad in this sentence, Croatian needs a finite verb, not an infinitive.

• govoriti = to speak (infinitive)
• govori = she speaks / is speaking (3rd person singular present)

So:

• učiteljica govori = the teacher speaks / is speaking

The clause kad učiteljica govori polako means when the teacher speaks slowly.

Croatian has no articles, so there is no separate word for the or a/an.

That means učiteljica can mean:

• a teacher
• the teacher

The exact meaning depends on context. In this sentence, English naturally uses the teacher, because it sounds like a specific teacher already known in the situation.

Polako is an adverb, and here it means slowly.

It describes how the teacher speaks:

• govori polako = speaks slowly

This is why Croatian uses polako, not an adjective form. You need an adverb to modify a verb.

Compare:

• polagan učitelj = a slow/calm teacher → adjective
• učitelj govori polako = the teacher speaks slowly → adverb

Yes, sporo can also mean slowly, and in many contexts it works:

• učiteljica govori sporo

But polako is very natural in speech when talking about someone speaking slowly, moving carefully, or doing something at a gentle pace.

Very roughly:

• polako often suggests slowly / gently / taking your time
• sporo focuses more directly on not fast

In this sentence, polako sounds very natural.

Croatian often uses the present tense for general truths, habits, or repeated situations.

This sentence does not necessarily mean one single moment right now. It can mean something like:

• I understand Croatian more and more whenever the teacher speaks slowly
• I’m understanding Croatian better and better when the teacher speaks slowly

So the present tense works well for a repeated or general situation.

No, Croatian word order is fairly flexible.

The sentence as given:

• Sve više razumijem hrvatski kad učiteljica govori polako.

is natural and emphasizes sve više early.

But you could also say:

• Kad učiteljica govori polako, sve više razumijem hrvatski.

That version puts the when clause first. The meaning stays basically the same, but the focus shifts slightly.

In this sentence, sve više mainly modifies razumijem.

So the idea is:

• I understand more and more

Not:

• the teacher speaks more and more slowly

If you wanted that meaning, Croatian would need a different structure, for example something like:

• Učiteljica govori sve sporije / sve polaganije.
  = The teacher speaks more and more slowly.

Yes, very much.

Even though the literal structure is I understand Croatian more and more when the teacher speaks slowly, the phrase sve više razumijem hrvatski naturally suggests growing ability or progress.

So the sentence gives both ideas:

• understanding happens better under a certain condition
• the speaker’s comprehension is gradually improving

That is one reason sve više is important here.