Hrvatski najbolje učim ujutro.

In Croatian, the subject pronoun is often omitted because the verb ending already tells you who the subject is.

• učim = I learn / I study
• the ending -im already shows I

So Hrvatski najbolje učim ujutro naturally means I learn/study Croatian best in the morning without needing ja.

You can add ja for emphasis, but it is not necessary:

• Ja hrvatski najbolje učim ujutro. = I learn Croatian best in the morning.

Here hrvatski is an adjective being used on its own to mean the Croatian language.

It is basically short for:

• hrvatski jezik = the Croatian language

After verbs like učiti, govoriti, razumjeti, Croatian often leaves out jezik and just uses the adjective:

• Učim hrvatski. = I’m learning Croatian.
• Govori engleski. = He/She speaks English.

So in this sentence, Hrvatski means Croatian as a language.

They usually are lowercase in Croatian. It is capitalized here only because it is the first word of the sentence.

So:

• Hrvatski najbolje učim ujutro. — capitalized because it starts the sentence
• Najbolje učim hrvatski ujutro. — lowercase in the middle of the sentence

This is an important difference from English. In Croatian, language names are normally lowercase:

• hrvatski
• engleski
• njemački

Because it is the direct object of učim, and in this pattern the language name appears in the accusative.

Since hrvatski here stands for hrvatski jezik, it behaves like a masculine inanimate form. For masculine inanimate nouns/adjectives, the accusative singular is the same as the nominative singular.

So:

• nominative: hrvatski
• accusative: hrvatski

That is why you get:

• Učim hrvatski.
• not Učim hrvatskog.

You would expect -og in some other grammatical contexts, but not here.

Najbolje is an adverb, and it means best.

It modifies the verb učim, so it tells you how well the action is done:

• najbolje učim = I learn best

Why not najbolji?

Because:

• najbolji is an adjective: the best boy/book/student, etc.
• najbolje is the adverb form: best when describing a verb

Compare:

• On je najbolji učenik. = He is the best student.
• On najbolje uči. = He studies best.

Ujutro means in the morning.

It is a common time expression used as an adverb:

• Ujutro radim. = I work in the morning.
• Najbolje učim ujutro. = I learn best in the morning.

So the sentence says that morning is the time when the speaker learns Croatian best.

For the meaning in the morning, the normal form is ujutro as one word.

So:

• ujutro = in the morning

Writing u jutro is generally not the standard way to express this everyday time meaning.

A learner should treat ujutro as a set expression, similar to:

• popodne = in the afternoon
• navečer = in the evening

Yes, you could. Croatian word order is much more flexible than English word order.

This sentence can be rearranged depending on what you want to emphasize:

• Hrvatski najbolje učim ujutro.
  Focus may feel slightly stronger on Croatian
• Najbolje učim hrvatski ujutro.
  A very natural order
• Ujutro najbolje učim hrvatski.
  Stronger focus on in the morning

The basic meaning stays the same. The differences are mostly about emphasis and information flow.

It can mean either, depending on context.

• učiti can mean to learn
• it can also mean to study

So this sentence could naturally be understood as:

• I learn Croatian best in the morning or
• I study Croatian best in the morning

If the meaning has already been given to the learner, then the grammar stays the same either way.

Učim is present tense, first person singular, from the imperfective verb učiti.

So grammatically it means:

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

depending on context.

Because it is imperfective, it works well for:

• habits
• routines
• ongoing activity

In this sentence, it most naturally describes a general habit:

• I learn/study Croatian best in the morning

A perfective verb such as naučiti would not fit the same way here, because naučiti is more like to learn successfully / to master rather than the ongoing process of studying.

Croatian does not have articles like a, an, and the.

So Croatian simply says:

• Učim hrvatski.
• literally: I learn Croatian

and

• ujutro
• literally: in morning, but naturally translated as in the morning

English needs articles, but Croatian does not. That is completely normal and one of the big structural differences between the two languages.