Learn Hardcore Croatian: Tijekom dana radim u uredu, a navečer učim hrvatski. - During the day I work in the office, and in the evening I study Croatian.

tijekom dana – shorter, very natural and common in both speech and writing.
za vrijeme dana – a bit longer, sometimes feels more formal or heavier in style.

In everyday speech, tijekom dana is more typical. There’s also po danu (“by day / in the daytime”), which is a bit more colloquial.

There’s no “the” in tijekom dana, u uredu, navečer. How do you say “the” in Croatian?

Croatian has no articles (no “a/an” and no “the”).

Whether something is definite or indefinite is understood from context, word order, and sometimes from additional words (like taj = that, ovaj = this, etc.), but you don’t mark it with a separate word like the.

So:

tijekom dana can mean “during the day” or “during a day”, depending on context.
u uredu = “in the office / in an office”
navečer = “in the evening” (English needs “the”, Croatian doesn’t)

You don’t need to translate “the” explicitly.

Why is it u uredu and not u ured?

Because u behaves differently depending on whether you talk about:

location (where?) – use locative case
movement into (where to?) – use accusative case

Here we have a location: “I work in the office.” So we use the locative:

ured (nominative) → u uredu (locative) = in the office
u uredu radim – I work in the office

If there were movement into the office, you’d use the accusative:

Idem u ured. – I’m going to the office.

So:

u uredu (locative) = in the office
u ured (accusative) = to the office

Could I say na poslu instead of u uredu? What’s the difference?

Yes, both are correct, but they’re not identical in meaning.

u uredu = in the office – it suggests an office as a place/room.
na poslu = at work – more general; it’s about being at your workplace, not necessarily in an office.

So:

Tijekom dana radim u uredu. – During the day I work in the office.
Tijekom dana radim na poslu. – During the day I’m at work.

Use whichever best matches what you want to say.

What’s the difference between navečer, uvečer, and večer?

večer is a noun: evening
- Ova večer – this evening
- U petak navečer – on Friday evening
navečer and uvečer are adverbs: in the evening / at nightfall
- Navečer učim hrvatski. – In the evening I study Croatian.
- Uvečer čitam knjige. – I read books in the evening.

navečer and uvečer are near-synonyms; both are standard and both mean “in the evening”. Usage preferences can be regional or personal, but both are fine in standard Croatian.

You normally don’t say u večer in this meaning; you use navečer/uvečer instead.

Why isn’t there a subject pronoun ja (“I”) before radim and učim? Is Ja radim wrong?

Croatian verb endings already show the subject, so the pronoun is usually omitted:

radim – I work
učiš – you (sg.) study
učimo – we study

So Radim u uredu. already means “I work in the office.”

Ja radim is not wrong; you use it for emphasis or contrast:

Ja radim u uredu, a ona radi od kuće.
I work in the office, but she works from home.

In your neutral sentence, it’s more natural without ja.

Why is it učim hrvatski and not učim hrvatski jezik? Are both correct?

Both are correct.

učim hrvatski jezik – literally “I’m learning the Croatian language.”
učim hrvatski – “I’m learning Croatian.”

In Croatian, language names are adjectives (hrvatski, engleski, njemački, …), and they’re very often used on their own as nouns, especially with verbs like učiti (to learn), govoriti (to speak), znati (to know):

Učim hrvatski. – I’m learning Croatian.
Znam engleski. – I know English.
Govoriš li njemački? – Do you speak German?

Grammatically, hrvatski here is accusative masculine singular (object of učim), but it happens to look the same as the nominative form.

Can I change the word order, for example: Radim u uredu tijekom dana or Učim navečer hrvatski?

Yes, Croatian word order is quite flexible, especially for time and place phrases. Your sentence:

Tijekom dana radim u uredu, a navečer učim hrvatski.

Possible variants (all correct, with slightly different emphasis):

Radim u uredu tijekom dana, a navečer učim hrvatski.
Radim u uredu tijekom dana, a hrvatski učim navečer.
U uredu radim tijekom dana, a navečer učim hrvatski.
Navečer učim hrvatski, a tijekom dana radim u uredu.

General tendencies:

Time expressions often appear at the beginning: Tijekom dana…, Navečer…
Information you want to emphasize often goes closer to the end of the clause.

There are preferences for naturalness, but all these options are grammatical.

What does the conjunction a express here, and why not use i or ali? Is the comma before a necessary?

In this sentence:

… radim u uredu, a navečer učim hrvatski.

a expresses a mild contrast or a shift:

During the day (one situation) I do X, and/whereas in the evening (another situation) I do Y.

Rough comparison:

i = “and” (simply adds things, no contrast)
a = “and/but, whereas” (contrast or parallel difference)
ali = “but” (stronger opposition)

So:

Radim u uredu i navečer učim hrvatski. – I work in the office and in the evening I study Croatian. (just listing actions)
Radim u uredu, a navečer učim hrvatski. – I work in the office, whereas in the evening I study Croatian. (day vs evening contrast)

About the comma:
Yes, in standard Croatian, you normally put a comma before “a” when it joins two clauses:

Radim u uredu, a navečer učim hrvatski. ✅

Are radim and učim here like English “I work / I study” (present simple)? What about aspect?

Yes. Radim and učim are both:

present tense
imperfective aspect

They describe habitual / regular actions:

Tijekom dana radim u uredu – During the day I (usually) work in the office.
Navečer učim hrvatski – In the evenings I (usually) study Croatian.

If you wanted to talk about completing the learning (result), you’d use a perfective verb like naučiti:

Želim naučiti hrvatski. – I want to learn (to have learned) Croatian.

But for ongoing, repeated activities as a routine, the imperfective raditi, učiti in the present are exactly what you need.