Learn Hardcore Croatian: Učeći navečer u tihoj sobi, ona ozbiljno misli na ispit. - Studying in the evening in a quiet room, she thinks seriously about the exam.

In this sentence it functions as an adverbial participial clause – it tells us when / under what circumstances she thinks about the exam: while studying.

You could roughly paraphrase it as:

Dok uči navečer u tihoj sobi, ona ozbiljno misli na ispit. (While she is studying in the evening in a quiet room, she seriously thinks about the exam.)

Why is there a comma after Učeći navečer u tihoj sobi?

In Croatian, a participial clause like Učeći navečer u tihoj sobi is treated similarly to a subordinate clause. It’s separated from the main clause by a comma.

Structure:

Participial clause: Učeći navečer u tihoj sobi,
Main clause: ona ozbiljno misli na ispit.

You must write that comma; without it, the sentence is considered incorrectly punctuated.

Could we say Ona učeći navečer u tihoj sobi ozbiljno misli na ispit without the comma?

Grammatically, that word order is possible but it sounds awkward and heavy in standard style. The natural way is:

Učeći navečer u tihoj sobi, ona ozbiljno misli na ispit.
or
Dok navečer uči u tihoj sobi, ona ozbiljno misli na ispit.

Croatian strongly prefers either:

a clear participial phrase + comma
or a finite clause with dok (+ comma)

Putting the participle inside the main clause as in your version is unusual and not typical in careful standard Croatian.

Why is navečer used, and not u večer or just večer?

Navečer is an adverb meaning in the evening / evenings. It’s a very common, idiomatic way to express time:

navečer ≈ in the evening, at night (in the evening hours)
uvečer also exists and is close in meaning; in many contexts they’re interchangeable.

You generally don’t say u večer in this sense. Instead you use:

navečer / uvečer – time adverbs: in the evening
ove večeri – this evening
jedne večeri – one evening

So Učeći navečer… is completely natural and idiomatic.

What case is u tihoj sobi and how do we know?

U tihoj sobi is in the locative singular:

Preposition: u (in)
Noun: soba (room), feminine
- Locative singular of soba: sobi
Adjective: tih (quiet)
- Feminine locative singular: tihoj

So:

tihoj (fem. loc. sg.)
sobi (fem. loc. sg.)

Croatian uses u + locative to express location in / inside something:

u gradu – in the city
u školi – in school
u tihoj sobi – in a quiet room

Why does the adjective appear as tihoj and not tiha or tiho?

The adjective tih (quiet) must agree with soba in gender, number and case:

soba – feminine, singular, locative (sobi)
Adjective tih → feminine, singular, locative: tihoj

Forms of tih (just the ones we need):

Nominative fem. sg.: tiha (quiet room – tiha soba)
Locative fem. sg.: tihoj (in a quiet room – u tihoj sobi)

So tihoj is required here because of the locative after u.

Why do we say ona ozbiljno misli na ispit and not just ozbiljno misli na ispit? Is ona necessary?

Croatian is a pro‑drop language: you can usually omit subject pronouns because the verb ending shows the person.

So both are correct:

Ona ozbiljno misli na ispit.
Ozbiljno misli na ispit.

Including ona:

adds slight emphasis on she (as opposed to someone else), or
is simply stylistic, especially after a longer introductory phrase.

In neutral, everyday speech, the version without the pronoun is often preferred unless you want that emphasis or extra clarity.

What is the difference between misliti na and misliti o, and why is it na ispit here?

Both misliti na and misliti o translate as to think about, but they have different nuances:

misliti na + accusative
- Focus on having something in mind, remembering, being preoccupied with it.
- Ona misli na ispit. – The exam is on her mind; she’s concerned about it.
misliti o + locative
- Focus on having an opinion / view / judgement about something.
- Što misliš o ispitu? – What do you think of the exam? (what’s your opinion?)

In your sentence, she is preoccupied with the exam, so misliti na ispit (accusative) is the correct choice.

Why is ispit in the form ispit, not something like ispitu?

Because with misliti na, the noun takes the accusative case:

Preposition: na (+ accusative here, by verb government)
Noun: ispit (masculine)
- Nominative sg.: ispit
- Accusative sg.: ispit (same form as nominative for most masculine inanimate nouns)

So na ispit is na + accusative, required by the verb phrase misliti na [koga/što].

Can the adverb ozbiljno be placed in other positions in the sentence?

Yes. Ozbiljno (seriously) is quite flexible. All of these are grammatical, with only small differences in emphasis:

Ona ozbiljno misli na ispit. (neutral)
Ona misli ozbiljno na ispit. (slightly emphasizes the manner of thinking)
Ozbiljno ona misli na ispit. (stresses that she really does think seriously)

In your complete sentence, the most natural is:

Učeći navečer u tihoj sobi, ona ozbiljno misli na ispit.

You could also say:

Učeći navečer u tihoj sobi, ona misli ozbiljno na ispit.

but moving ozbiljno too far away can sound a bit marked or stylistic.

Could we replace Učeći navečer u tihoj sobi with a clause using dok? Would it mean the same?

Yes. A very natural equivalent is:

Dok navečer uči u tihoj sobi, ona ozbiljno misli na ispit.

This has essentially the same meaning:

Učeći navečer u tihoj sobi… – While studying in the evening in a quiet room…
Dok navečer uči u tihoj sobi… – While she is studying in the evening in a quiet room…

The dok + finite verb version is more colloquial and common in speech; the učeći participial construction feels a bit more bookish/formal or stylistically elevated.

Is there any aspectual nuance with učiti and misliti here (imperfective vs perfective)?

Yes:

učiti – imperfective (to study, ongoing/habitual action)
misliti – imperfective (to think, ongoing process)

Using imperfective forms here emphasizes ongoing, repeated or continuous actions:

She is (in general) studying in the evening in a quiet room.
While she does that, she continually / repeatedly thinks about the exam.

If you changed them to perfective (e.g. naučiti, pomisliti), the meaning would shift to one‑time completed events, which would not fit the idea of a general habitual situation in this sentence.