Mnogo učim hrvatski ovaj tjedan.

Croatian often drops subject pronouns because the verb ending already shows who is doing the action.

Here, učim means I study / I am studying / I learn, so ja is not necessary. You could say Ja mnogo učim hrvatski ovaj tjedan, but that usually adds emphasis, like I am studying a lot this week.

Učim is the 1st person singular present tense of the verb učiti.

So:

• učiti = to learn, to study
• učim = I learn / I study / I am studying

Croatian present tense often covers several English forms, so učim can correspond to both I study and I am studying, depending on context.

Not necessarily.

In this sentence, the present tense describes what is going on during this period, not only at this exact second. Because of ovaj tjedan (this week), the meaning is more like I’m studying Croatian a lot this week or I’ve been studying Croatian a lot this week.

So Croatian present tense can be broader than English right now present.

Because učiti is the normal verb for learning/studying something, especially a language or school subject.

• učim hrvatski = I’m learning/studying Croatian
• studiram usually means I study at university or I am a university student, often with a field:
  • Studiram medicinu = I study medicine at university

So for languages, učim is the natural choice.

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

• učiti = imperfective, focusing on the process
• naučiti = perfective, focusing on the result or completion

This sentence talks about an ongoing activity during the week, so učim fits well.

If you used naučiti, the idea would be more like to learn/master Croatian, not just to spend time studying it. That changes the meaning a lot.

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

So:

• hrvatski = Croatian (language)
• engleski = English
• njemački = German

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

Croatian does capitalize:

• Hrvatska = Croatia
• Hrvat = a Croat

But the language/adjective hrvatski is lowercase unless it starts the sentence.

Here hrvatski is the direct object of učim, so it is functioning like the Croatian language.

You can think of it as a shortened form of:

• učim hrvatski jezik = I study the Croatian language

In the sentence, hrvatski is in the accusative, but for a masculine inanimate form like this, the accusative looks the same as the nominative. So the form stays hrvatski.

Yes, both are possible.

• ovaj tjedan uses the accusative as a time expression
• ovog tjedna uses the genitive, which is also very common for time expressions

In this sentence, both would naturally mean this week:

• Mnogo učim hrvatski ovaj tjedan.
• Mnogo učim hrvatski ovog tjedna.

The difference is small here, and learners will hear both patterns.

Croatian word order is more flexible than English word order.

Putting mnogo first gives it extra emphasis, as if highlighting a lot:

• Mnogo učim hrvatski ovaj tjedan. = emphasis on how much
• Ovaj tjedan mnogo učim hrvatski. = emphasis on this week

So the sentence is not strange; it just reflects Croatian flexibility and emphasis. The meaning stays basically the same, but the focus shifts a little.

Yes, in this kind of sentence they are very close in meaning.

• mnogo = a lot, much
• puno = a lot, much

So you could also say:

• Puno učim hrvatski ovaj tjedan.

In everyday speech, puno is often very common, while mnogo can sound a bit more formal or emphatic in some contexts. But both are normal and correct here.