Learn Hardcore Croatian: On je jedini student koji večeras uči hrvatski u parku. - He is the only student who is studying Croatian in the park tonight.

jedan student = one student (just a quantity; there might be others doing the same thing)
jedini student = the only student (no other student is doing that)

In the sentence On je jedini student..., the point is that he is the only student who is studying Croatian in the park tonight, so jedini is the correct choice.

What case is student in, and why?

student is in the nominative singular.

Pattern:

Subject: On (nominative)
Verb: je
Predicate noun (what he is): jedini student (also nominative)

In Croatian, the noun after biti (to be) usually stays in the nominative when it describes or identifies the subject:

On je student.
Ona je učiteljica.
Oni su prijatelji.

Why is it koji and not koja or koje?

koji is a relative pronoun that must agree with the noun it refers to in:

gender
number
case

Here it refers to student:

student = masculine, singular, nominative
so you use koji = masculine, singular, nominative

If it referred to:

djevojka (feminine): jedina djevojka koja večeras uči hrvatski…
djeca (plural): jedina djeca koja večeras uče hrvatski…

Why is the verb uči (singular) and not uče (plural) after koji?

Because koji refers back to jedini student, which is singular.

So the verb inside the relative clause agrees with student (singular), not with some hidden plural idea like "students":

On je jedini student koji večeras uči hrvatski.
(He is the only student who studies…)

Compare:

Oni su studenti koji večeras uče hrvatski.
(They are students who are studying… → plural uče)

Why is je in the second position: On je jedini student…?

je is a clitic (a short, unstressed form of biti) and Croatian has a strong tendency to put such words in the second position in the clause (the so‑called Wackernagel position).

So:

On je jedini student… (standard)
You can change the first word, but je still tries to stay second:
- Samo on je jedini student…
- Večeras je on jedini student…

You would not normally say On jedini student je… in standard Croatian.

Can we omit On and just say Je jedini student koji večeras uči hrvatski u parku?

You cannot start the sentence with je like that. Croatian is pro‑drop (you can drop the subject pronoun), but the clitic je still wants to stay in second position.

Correct options:

On je jedini student… (with pronoun)
On je jedini student… → you can drop On only if you change the order:
- Jedini je student koji večeras uči hrvatski u parku.
- Jedini student koji večeras uči hrvatski u parku je on.

So you can drop On, but you cannot just start with Je.

Why is it u parku and not u park?

Because here he is in the park (location, static), not going into the park (movement).

u parku → locative case (where something is)
u park → accusative case (movement into a place)

So:

On uči hrvatski u parku.
He is studying Croatian in the park (already there).
On ide u park učiti hrvatski.
He is going to the park to study Croatian.

What exactly is hrvatski here: a noun or an adjective? Why is it not hrvatski jezik?

Grammatically, hrvatski is an adjective (Croatian), but it’s being used as a noun to mean the Croatian language.

Both forms are fine:

uči hrvatski = studies Croatian (language)
uči hrvatski jezik = studies the Croatian language (more explicit, a bit heavier)

This is very similar to English:

“He is learning Croatian.”
(adjective used as a noun)
“He is learning the Croatian language.”

Why is hrvatski not visibly declined? Shouldn’t the accusative look different?

For masculine singular adjectives describing inanimate things, the nominative and accusative usually look the same:

nominative: hrvatski (as an adjective)
accusative (inanimate): also hrvatski

So in:

On uči hrvatski.

hrvatski is actually accusative masculine singular, but its form matches the nominative.

If it referred to a person (animate masculine), you would see a change:

On vidi hrvatskog učitelja.
(accusative hrvatskog for an animate masculine noun)

Could we say uči se hrvatski instead of uči hrvatski?

You can say uči se hrvatski, but it has a different focus:

On večeras uči hrvatski u parku.
He (personally) is studying Croatian.
Večeras se u parku uči hrvatski.
Croatian is being studied in the park tonight.
(more impersonal / passive‑like; focus on the activity, not the subject)

In your sentence, the goal is to say he is the only student who is studying, so uči hrvatski is the natural choice.

Is the word order koji večeras uči hrvatski u parku fixed, or can we move words around?

Croatian word order is relatively flexible, but changes can affect emphasis.

All of these are grammatically fine:

koji večeras uči hrvatski u parku (neutral)
koji večeras u parku uči hrvatski (slight emphasis on location before the verb)
koji u parku večeras uči hrvatski (emphasis on in the park tonight)

The original order is the most neutral and natural in everyday speech. Other orders are used to highlight particular parts of the sentence.

Could we say this in the future tense, and how would that change the sentence?

Yes, for example:

On je jedini student koji će večeras učiti hrvatski u parku.

Differences:

uči (present) → describes something arranged and understood as happening tonight, close future but seen as fixed/ongoing.
će učiti (future) → more explicitly future, focusing on the action that will happen.

Both are possible; context decides which is more natural. Using present for near future (with a time word like večeras) is very common in Croatian, just like in English:
“He’s studying Croatian in the park tonight.”

Why is the relative clause koji večeras uči hrvatski u parku after student and not closer to on?

The relative clause koji… always attaches to the noun it describes, not to the pronoun.

On je jedini student koji večeras uči hrvatski u parku.
→ the clause describes student (what kind of student?)
You cannot say: On koji večeras uči hrvatski u parku je jedini student.
This sounds very odd or wrong in standard Croatian.

Think of koji… as “who / that” in English – it sticks right after the noun it’s modifying:

“He is the only student who is studying Croatian tonight.”