Learn Hardcore Croatian: Učenik navečer uči hrvatski u tihoj sobi. - In the evening the pupil studies Croatian in a quiet room.

Učenik means pupil / school student, usually someone in primary or secondary school.
Student in Croatian is used mostly for university or college students.

So:

učenik = school-age learner (elementary / high school)
student = university-level learner

In this sentence, Učenik navečer uči… implies a school pupil, not a university student.

Why are there no articles like “the” or “a” before učenik and hrvatski?

Croatian has no articles (no “a/an/the”). The bare noun or adjective does the job.

Učenik can mean a student or the student, depending on context.
hrvatski here means Croatian (language); again, it could be understood as “Croatian”, “the Croatian language” etc.

Which English article you choose in translation is determined by context, not by a word in Croatian.

What form of the verb is uči, and how is učiti conjugated?

Uči is the 3rd person singular present tense of the verb učiti (“to study / to learn / to teach”).

Present tense of učiti:

ja učim – I study / learn
ti učiš – you study / learn (sg., informal)
on/ona/ono uči – he/she/it studies / learns
mi učimo – we study / learn
vi učite – you study / learn (pl. or formal)
oni/one/ona uče – they study / learn

So Učenik uči = “The student studies / is studying”.

What’s the difference between učiti and naučiti?

Croatian uses aspect:

učiti is imperfective – it focuses on the process: “to study”, “to be learning”.
- Učenik navečer uči hrvatski. – He studies Croatian in the evenings (ongoing / habitual action).
naučiti is perfective – it focuses on the result: “to learn (finish learning), to have learned”.
- Učenik je naučio hrvatski. – The student has learned Croatian (he now knows it).

In your sentence, we talk about a regular activity, so učiti is the correct choice.

Why is it just hrvatski and not hrvatski jezik?

Literally, hrvatski jezik means “Croatian language”.

However, when talking about studying or speaking a language, Croatian often uses just the adjective:

učiti hrvatski – to learn Croatian (language)
govoriti hrvatski – to speak Croatian

Grammatically, hrvatski here is a masculine singular accusative adjective used as a noun: “Croatian (language)”.
You could say Učenik navečer uči hrvatski jezik, but it’s more formal / explicit; everyday speech prefers the shorter hrvatski.

What does navečer mean exactly, and is it different from večer?

večer is a noun: “evening”.
- Navečer idem kući. – In the evening I go home.
- Ova večer je lijepa. – This evening is beautiful.
navečer is an adverb meaning “in the evening / at night (in the evenings)”, usually for a time of day or habitual action.

So Učenik navečer uči… = “The student studies in the evening / in the evenings.”
You use navečer like an English time adverb: “in the evening”.

Why is it u tihoj sobi and not something like u tiha soba? What case is this?

After u (“in”) you get two possible cases:

Accusative – when there is movement into something:
- Idem u sobu. – I’m going into the room.
Locative – when something is in a place (location, no movement):
- Učim u sobi. – I study in the room.

In u tihoj sobi, the action happens in the room (no movement), so Locative is used:

tihoj – feminine singular locative of tiha (quiet)
sobi – feminine singular locative of soba (room)

That’s why you see -oj and -i endings: they are locative endings.

How do tihoj and sobi agree, and what are their base forms?

The base (dictionary) forms are:

tiha – “quiet” (feminine, nominative singular)
soba – “room” (feminine, nominative singular)

Croatian adjectives must agree with the noun in:

gender – both are feminine
number – both are singular
case – both are locative here

So we get:

nominative: tiha soba – a quiet room
locative: u tihoj sobi – in a quiet room

This agreement is why both words change their endings.

Can I drop Učenik and just say Navečer uči hrvatski u tihoj sobi?

Yes. Croatian is a pro‑drop language: you can often omit the subject pronoun or noun if it’s clear from context.

Navečer uči hrvatski u tihoj sobi. – In the evening, (he/she) studies Croatian in a quiet room.

From verb form uči (3rd person singular), we know it’s he/she/it; context would tell you who exactly. In isolation, it’s ambiguous, but in real conversations it’s usually clear.

Can I change the word order, for example: Učenik uči hrvatski navečer u tihoj sobi? Does it change the meaning?

Croatian word order is more flexible than English. All of these are grammatically fine:

Učenik navečer uči hrvatski u tihoj sobi.
Učenik uči hrvatski navečer u tihoj sobi.
Navečer učenik uči hrvatski u tihoj sobi.

The basic meaning stays the same. Changing order usually affects:

emphasis (what you highlight)
rhythm / style

Starting with Navečer emphasizes time (“In the evening, the student…”). Putting navečer later can make it sound slightly less prominent. But all are understood as the same situation.