Learn Hardcore Croatian: Stranac u parku mirno uči hrvatski. - The foreigner in the park is calmly studying Croatian.

Croatian has no articles like a/an or the. Nouns appear without articles, and the context tells you whether you should understand it as a foreigner, the foreigner, a park, or the park.

Stranac can mean a foreigner or the foreigner.
park (here in the form parku) can mean a park or the park.

If you really need to make it more specific, Croatian uses other words, e.g.:

onaj stranac – that foreigner
taj park – that/the park (already known from context)

What case is stranac in, and why?

Stranac is in the nominative singular, masculine.

The nominative is the case used for the subject of the sentence (the “doer” of the action).
Since the foreigner is the one doing the learning, stranac must be nominative.

Basic singular forms of stranac (“foreigner”):

Nominative: stranac – (who?) a foreigner
Genitive: stranca – (of whom?) of a foreigner
Dative: strancu – (to whom?) to a foreigner
Accusative: stranca – (whom?) a foreigner (object)
Locative: strancu – (about/at whom?) about a foreigner
Instrumental: strancem – (with whom?) with a foreigner

In our sentence it’s the subject, so we use stranac (nominative).

What case is u parku, and why is it parku instead of park?

U parku is locative singular.

In Croatian, prepositions usually require a specific case. With u:

u + accusative = into (movement into something)
- Idem u park. – I’m going to (into) the park.
u + locative = in / at (location, no movement)
- Sam u parku. – I am in the park.

In your sentence there’s no movement; it’s just stating where the foreigner is learning, so you need locative: u parku (in the park).

Singular forms of park:

Nominative: park – (what?) park
Genitive: parka
Dative: parku
Accusative: park
Locative: parku
Instrumental: parkom

That’s why you see parku after u here.

Is the word order Stranac u parku mirno uči hrvatski fixed, or can I change it?

The word order is not fixed; Croatian has fairly flexible word order. All of these are grammatically correct:

Stranac u parku mirno uči hrvatski.
Stranac mirno uči hrvatski u parku.
U parku stranac mirno uči hrvatski.
U parku mirno uči hrvatski stranac.

The basic neutral order (closest to English) is usually:

Subject – (place/time) – verb – object – (place/time)
Stranac u parku mirno uči hrvatski.

Changing the order mainly changes emphasis or what is “new” information:

Starting with U parku… emphasizes the location: In the park, (it is) a foreigner who is calmly learning Croatian.
Putting hrvatski earlier could emphasize what he’s learning.

For everyday speech, all of these versions will usually be understood the same; word order becomes more important when you want to stress something specific.

What exactly does mirno mean, and where can it go in the sentence?

Mirno is an adverb meaning calmly, peacefully, quietly. It comes from the adjective miran (calm, peaceful).

In this sentence it describes how he is learning:

mirno uči = is calmly learning / is studying quietly

Adverbs like mirno are pretty mobile:

Stranac u parku mirno uči hrvatski.
Stranac u parku uči mirno hrvatski.
Stranac mirno uči hrvatski u parku.
Stranac u parku uči hrvatski mirno.

The most natural is usually to put it right before the verb (mirno uči), but putting it elsewhere is still understood; it may sound a bit more marked or emphatic in some positions.

What tense and aspect is uči, and how would I say “is learning” vs. “learns”?

Uči is:

3rd person singular
present tense
of the verb učiti (to learn, to study, to teach),
which is imperfective (focuses on an ongoing or repeated action, not on completion).

Croatian doesn’t distinguish between he learns and he is learning with a separate tense like English does. Uči can mean:

He learns Croatian (in general).
He is learning Croatian (right now / these days).

The exact meaning comes from context.

If you wanted to emphasize that he finished learning, you’d usually use the perfective verb naučiti:

Naučio je hrvatski. – He has learned Croatian / He learned Croatian (and now he knows it).

Why is it uči hrvatski and not something like uči hrvatskog?

After učiti in the sense to learn a language, the language name behaves like an inanimate object in the accusative, and for masculine inanimate, the accusative is identical to the nominative.

učiti + (koji jezik?) – to learn which language?
uči hrvatski – he is learning Croatian (language)

Since hrvatski here is inanimate (it’s a language, not a person), its:

Nominative: hrvatski
Accusative: hrvatski

The form hrvatskog would be genitive/accusative for a masculine animate noun (like a Croatian man), but that’s not what’s meant here.

Is hrvatski an adjective or a noun here, and what case is it?

Formally, hrvatski is an adjective meaning Croatian.

However, in this sentence it is used as a noun: it stands for hrvatski jezik (the Croatian language). This is very common for languages:

uči hrvatski – he is learning Croatian (language)
govori engleski – he speaks English
čitam njemački – I read German (in German)

Case:

It’s the direct object of uči, so it is in the accusative.
For inanimate masculine adjectives, accusative = nominative, so the form is hrvatski in both cases.

Could I say uči se hrvatski instead? What’s the difference between učiti and učiti se?

Both exist, but they’re used differently.

učiti + accusative object (what is being learned)
- Stranac u parku mirno uči hrvatski.
  The foreigner is calmly learning Croatian (language).
- Focus: what he is learning / studying.
učiti se (reflexive) – roughly to study / to learn (for something)
Common with za + accusative or a test/exam:
- Uči se za ispit. – He is studying for the exam.
- Djeca se uče čitati. – Children are learning to read.

If you say Stranac se uči hrvatski, it sounds odd/wrong in standard Croatian. You would say either:

Stranac uči hrvatski. – He is learning Croatian. (correct, natural)
Stranac se uči hrvatskom jeziku. – rarer, more like He is being trained/taught in the Croatian language; feels formal and unusual in everyday speech.

For normal “He is learning Croatian”, use učiti + hrvatski (no se).

Why is there no word for “he” in the sentence?

Croatian is a “pro-drop” language: subject pronouns (ja, ti, on, ona, etc.) are often omitted because the verb ending already tells you who the subject is.

uči is 3rd person singular (he/she/it).
From context (stranac) we know it means he.

You could say:

On mirno uči hrvatski u parku. – He is calmly learning Croatian in the park.

But usually, if the subject (here stranac) is present in the sentence, on is unnecessary and sounds redundant.

How do you pronounce uči, especially the letter č, and how is it different from ući?

uči (from učiti) is pronounced roughly like:

[OO-chee], where:
- u = like oo in food
- č = like ch in church (stronger, always hard)
- i = like ee in see

č is a postalveolar affricate /tʃ/, always spelled and pronounced the same.

Compare with ući:

ući (from ući = to enter) is pronounced roughly:
- [OO-chee] but with a softer “ch” sound.
- The letter ć is a softer consonant /tɕ/, a bit between English t and ch.
Meanings:
- uči – he/she learns / is learning
- ući – to enter / to go in

To many English ears, č and ć may sound similar at first, but they are distinct sounds and always spelled differently.

Is stranac always masculine? How would you say “a female foreigner”?

Stranac is a masculine noun (grammatically and typically referring to a male).

For a female foreigner, you use:

strankinja – a (female) foreigner

Examples:

Stranac u parku uči hrvatski. – A (male) foreigner in the park is learning Croatian.
Strankinja u parku uči hrvatski. – A (female) foreigner in the park is learning Croatian.

Can I say Stranac u parku mirno uči hrvatski jezik, or is that wrong?

It’s correct, just a bit more explicit and slightly more formal or explanatory:

Stranac u parku mirno uči hrvatski.
– The foreigner in the park is calmly learning Croatian.
Stranac u parku mirno uči hrvatski jezik.
– The foreigner in the park is calmly learning the Croatian language.

In everyday speech, people almost always drop “jezik” and just say hrvatski when they mean the language.