Learn Hardcore Croatian: Doktor danas radi u bolnici. - The doctor is working at the hospital today.

Doktor danas radi u bolnici. = “The doctor” or “A doctor,” depending on context.
To make it clearly definite, use a demonstrative: Taj/ovaj doktor danas radi u bolnici. (That/this doctor…)
To make it clearly indefinite: Jedan/neki doktor danas radi u bolnici. (A/one/some doctor…)

Does radi mean “works” or “is working”?

Both. The present tense of the imperfective verb raditi covers English simple and progressive:

Doktor danas radi u bolnici. = “The doctor works at the hospital today” / “is working at the hospital today.” There is no special “be + -ing” form in Croatian. If you mean mere presence, not working, say: Doktor je danas u bolnici. (The doctor is in the hospital today.)

How do I conjugate raditi (to work) in the present?

ja radim
ti radiš
on/ona/ono radi
mi radimo
vi radite
oni/one/ona rade Negation: put ne before the verb, e.g., ne radi (is not working/doesn’t work).

Why is it u bolnici and not u bolnica or u bolnicu?

After u:

Use locative for location: u bolnici = in/at the hospital.
Use accusative for motion into: u bolnicu = into/to the hospital. So: Radi u bolnici, but Ide u bolnicu.

What case is bolnici, and how does bolnica decline?

Bolnici is locative singular. Key singular forms:

Nominative: bolnica
Genitive: bolnice
Dative: bolnici
Accusative: bolnicu
Locative: bolnici
Instrumental: bolnicom

Can I say na bolnici?

No; use u bolnici. Croatian splits “at/in” roughly as:

u for being inside/enclosed places (u kući, u školi, u bolnici).
na for surfaces, events, many institutions (na poslu, na fakultetu, na sudu). There are fixed expressions, so learn them case by case. For hospitals, it’s u bolnici.

Where can I put danas in the sentence?

Word order is flexible for emphasis:

Neutral/common: Doktor danas radi u bolnici.
Emphasizing “today”: Danas doktor radi u bolnici.
End focus: Doktor radi u bolnici danas. Meaning stays the same; you just shift the focus.

How do I make a yes/no question?

Use li after the verb, or just rise your intonation:

Radi li doktor danas u bolnici? (Is the doctor working at the hospital today?)
Colloquial: Doktor danas radi u bolnici? (rising intonation) Be careful: Je li doktor danas u bolnici? asks if he is there (present), not necessarily working.

How do I negate the sentence?

Place ne before the verb:

Doktor danas ne radi u bolnici. (The doctor isn’t working at the hospital today.)

How do I say “to the hospital,” “in the hospital,” and “from the hospital”?

To the hospital (motion in): u bolnicu
In/at the hospital (location): u bolnici
From the hospital: iz bolnice Examples: Idem u bolnicu. Radim u bolnici. Dolazim iz bolnice.

Does doktor always mean a medical doctor?

Everyday Croatian uses doktor for a physician, but it can also mean a PhD. The specifically medical term is liječnik (male) / liječnica (female):

Liječnik danas radi u bolnici. (More formal/precise.)

How do I say it for a female doctor?

Use the feminine form:

Doktorica danas radi u bolnici. Or with the formal medical term:
Liječnica danas radi u bolnici.

What are the plural forms?

Subject plural: Doktori danas rade u bolnici. / Liječnici danas rade u bolnici.
“In hospitals” (plural): u bolnicama.

Can I drop the subject like in Spanish/Italian?

Yes. Croatian is pro-drop. If the subject is clear from context:

Danas radi u bolnici. You can use a pronoun for emphasis or contrast: On danas radi u bolnici. (He, specifically, is working today.)

Why not say Doktor je danas radi u bolnici?

Because je (is) doesn’t form a progressive. Use either:

Doktor danas radi u bolnici. (He is working.) or
Doktor je danas u bolnici. (He is at the hospital.)

Any quick pronunciation tips?

c = “ts” (so bolnici ≈ “bol-nee-tsee”).
u = “oo.”
r is tapped.
Roughly: Doktor “DOK-tor,” danas “DAH-nahs,” radi “RAH-dee,” u “oo,” bolnici “BOL-nee-tsee.”

How can I emphasize “today”?

Add particles:

Baš danas / Upravo danas (exactly/just today): Baš danas doktor radi u bolnici.
Tek danas (only today): Doktor tek danas radi u bolnici.

I’ve seen radi used as “because of.” Is that the same word?

It’s a different usage. Radi + genitive can mean “because of” (formal/literary), e.g., Radi kiše ostajemo doma. In your sentence, radi is the verb “works.”