Učenik navečer uči hrvatski u tihoj sobi.

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.

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.

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”.

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.

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.

• 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”.

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.

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.

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.

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.