Ovaj izraz čujem često u Zagrebu, ali ga ne znam dobro prevesti.

Because ovaj izraz is the direct object of čujem (I hear), so it needs the accusative case.

Here, izraz is a masculine inanimate noun. In Croatian, masculine inanimate nouns usually have the same form in the nominative and accusative singular, so:

• nominative: ovaj izraz = this expression
• accusative: ovaj izraz = this expression

That is why the form does not visibly change.

By contrast, ovog izraza would be genitive, not the correct case here.

Izraz usually means expression, phrase, or sometimes way of saying something.

In this sentence, it most naturally means:

• expression
• phrase
• turn of phrase

So the speaker is saying they often hear a certain local expression in Zagreb, but do not know how to translate it well.

Because čuti / čujem means to hear, while slušati / slušam means to listen.

That is an important difference:

• čujem = I hear it / I can hear it
• slušam = I am listening to it

In this sentence, the idea is that the speaker often hears the expression around Zagreb, not that they actively sit and listen to it.

So:

• Ovaj izraz čujem često... = I often hear this expression...

not:

• I often listen to this expression...

Yes, it could go somewhere else. Croatian word order is fairly flexible.

In the sentence:

• Ovaj izraz čujem često u Zagrebu...

često means often and modifies čujem.

Other natural word orders are possible, for example:

• Ovaj izraz često čujem u Zagrebu.
• Često čujem ovaj izraz u Zagrebu.

All of these are understandable. The differences are mostly about emphasis and style, not basic meaning.

The given sentence sounds natural and conversational.

Because u can take different cases depending on meaning:

• u + accusative = motion into somewhere
• u + locative = being in somewhere

Here the meaning is location, not movement:

• u Zagrebu = in Zagreb

Compare:

• Živim u Zagrebu. = I live in Zagreb.
• Idem u Zagreb. = I am going to Zagreb.

So Zagrebu is the locative singular form after u when talking about location.

Ga refers back to ovaj izraz.

So the sentence first says:

• Ovaj izraz = this expression

Then, in the second clause, instead of repeating ovaj izraz, Croatian uses the clitic pronoun ga:

• ali ga ne znam dobro prevesti = but I don’t know how to translate it well

So ga = it.

Grammatically, it is the accusative singular clitic form for a masculine noun here.

Because ga is a clitic, and Croatian clitics usually appear in the second position of the clause.

So in:

• ali ga ne znam dobro prevesti

the first element is ali (but), and the clitic ga comes right after it.

This is very typical Croatian word order for clitics.

You will often see this pattern:

• Ali ga ne razumijem.
• Imaš li ga?
• Ne mogu ga naći.

Clitics are short unstressed words, and they tend to occupy this special early position in the clause.

Yes, literally it is something like I do not know translate, but in natural English we say:

• I don’t know how to translate it
• I can’t translate it well
• I don’t know the best way to translate it

In Croatian, znati + infinitive can mean to know how to do something.

So:

• znam plivati = I know how to swim
• ne znam objasniti = I don’t know how to explain
• ne znam prevesti = I don’t know how to translate

You could also say:

• ali ga ne znam dobro prevesti
• ali ga ne znam kako dobro prevesti is not idiomatic
• ali ga ne znam prevesti is perfectly normal

If you want to make how explicit, Croatian more naturally uses:

• ali ne znam kako ga dobro prevesti

But the original sentence is very natural.

This is a matter of aspect.

• prevesti = perfective
• prevoditi = imperfective

In this sentence, the speaker is talking about achieving a translation of this specific expression, so the result matters. That is why prevesti is natural.

• ne znam ga dobro prevesti = I don’t know how to translate it well / I can’t come up with a good translation

If you used prevoditi, it would sound more like an ongoing or habitual activity of translating in general.

Compare:

• Volim prevoditi tekstove. = I like translating texts.
• Ne znam prevesti ovu riječ. = I don’t know how to translate this word.

So here, prevesti is the better choice.

Yes, dobro is the adverb well.

In this sentence, it means something like:

• accurately
• properly
• naturally
• in a good way

So ne znam dobro prevesti does not necessarily mean the speaker cannot translate it at all. It often means:

• they are not confident about the best translation
• no translation feels quite right
• they do not know how to render the local nuance well

That is a very common use of dobro in Croatian.

Yes. A word-for-word translation is roughly:

• This expression I hear often in Zagreb, but it I do not know well translate.

Natural English versions would be:

• I often hear this expression in Zagreb, but I don’t know how to translate it well.
• I hear this expression a lot in Zagreb, but I’m not sure how best to translate it.
• I often hear this expression in Zagreb, but I can’t really translate it properly.

The last two are especially natural if the idea is that the expression is culturally specific or hard to render in English.

Yes. Croatian allows several natural word orders. For example:

• Često čujem ovaj izraz u Zagrebu, ali ga ne znam dobro prevesti.
• Ovaj izraz često čujem u Zagrebu, ali ga ne znam dobro prevesti.
• U Zagrebu često čujem ovaj izraz, ali ga ne znam dobro prevesti.

All of these are acceptable. The main differences are emphasis:

• starting with Ovaj izraz emphasizes the expression
• starting with Često emphasizes frequency
• starting with U Zagrebu emphasizes location

The original sentence is perfectly natural, especially in speech.

It is fairly neutral and natural. It works well in normal conversation and also in informal writing.

A few points:

• izraz is a standard word
• čujem često u Zagrebu sounds conversational and natural
• ne znam dobro prevesti is common, idiomatic Croatian

So the sentence is not slangy, but it is not overly formal either. It sounds like something an educated speaker could easily say in everyday conversation.