Najčešće učim hrvatski navečer.

In Croatian, subject pronouns are often left out because the verb ending already shows who is doing the action.

• učim = I learn / I study
• the ending -im tells you it is first person singular

So Najčešće učim hrvatski navečer already means I usually study Croatian in the evening without needing ja.

You could add ja as Ja najčešće učim hrvatski navečer, but that usually adds emphasis, contrast, or clarity.

Najčešće means most often, and in natural English it is often translated as usually.

It is built like this:

• često = often
• češće = more often
• najčešće = most often

So it is the superlative adverb of često.

They are similar, but not identical:

• često = often
• najčešće = most often / usually
• obično = usually / ordinarily

In this sentence, najčešće suggests that evening is the time when this happens more than at other times.

So:

• Često učim hrvatski navečer = I often study Croatian in the evening.
• Najčešće učim hrvatski navečer = I study Croatian in the evening most often.
• Obično učim hrvatski navečer = I usually study Croatian in the evening.

In everyday English, najčešće and obično may both come out as usually, but the Croatian nuance is a little different.

Učim is the present tense of učiti.

Depending on context, it can mean:

• I learn
• I study
• I am learning
• I am studying

In this sentence, because of najčešće and navečer, the meaning is habitual: something you generally do at that time.

So here it is best understood as:

• I usually study Croatian in the evening or
• I most often learn/study Croatian in the evening

Because učiti and naučiti are different in aspect.

• učiti = to learn, to study; ongoing or repeated action
• naučiti = to learn, master, or finish learning; completed action

This sentence describes a repeated habit, so učiti is the correct choice.

• Najčešće učim hrvatski navečer = I usually study Croatian in the evening.
• Naučim hrvatski navečer would not fit well for this meaning, because it sounds like completing the learning.

Hrvatski is the direct object of učim, so it is in the accusative case.

What is slightly tricky is that hrvatski here really stands for hrvatski jezik = the Croatian language.

So the full idea is:

• Učim hrvatski jezik = I am learning the Croatian language.

Very often, jezik is simply left out, and you just say:

• Učim hrvatski

Because jezik is a masculine inanimate noun, its accusative singular looks the same as the nominative. That is why you see hrvatski, not a visibly different form.

In Croatian, names of languages are normally written with a lowercase letter.

So:

• hrvatski = Croatian
• engleski = English
• njemački = German

This is different from English, where language names are capitalized.

Navečer means in the evening.

It is an adverb of time, so you do not need a preposition in English-style structure.

You can also hear and use uvečer, which also means in the evening. In many contexts, both are natural.

So these are both possible:

• Najčešće učim hrvatski navečer
• Najčešće učim hrvatski uvečer

Both are understandable and common.

Yes. Croatian word order is more flexible than English word order.

The sentence starts with najčešće to foreground the idea of frequency. That sounds natural.

Other orders are also possible, for example:

• Najčešće učim hrvatski navečer
• Hrvatski najčešće učim navečer
• Navečer najčešće učim hrvatski

These all mean roughly the same thing, but the emphasis shifts slightly:

• starting with najčešće highlights frequency
• starting with hrvatski highlights what you study
• starting with navečer highlights the time

So the original sentence is natural, but not the only possible order.

The main letters to notice are:

• j = like English y in yes
• č = a harder ch sound, like ch in chocolate
• ć = a softer sound than č; many beginners pronounce it close to ch at first
• š = sh

So:

• učim has u + č + im
• najčešće contains both č and ć, plus š

A useful beginner approach is:

• pronounce č as ch
• pronounce š as sh
• try to make ć softer than č

Even if you do not fully master the č / ć difference right away, native speakers will usually still understand you.