Questions & Answers about Doktor kaže da se virus brzo širi u školi.
Because kaže is the 3rd person singular present form of kazati / reći in the sense of to say.
Here the subject is doktor = the doctor, which is singular, so the verb must also be singular:
- ja kažem = I say
- ti kažeš = you say
- on/ona kaže = he/she says
- oni kažu = they say
So Doktor kaže means The doctor says.
Da introduces a subordinate clause, very often corresponding to English that.
So:
- Doktor kaže = The doctor says
- da se virus brzo širi u školi = that the virus is spreading quickly in the school
In Croatian, da is extremely common after verbs like:
- reći / kazati = to say
- misliti = to think
- znati = to know
- vidjeti = to see
So this sentence is structured very naturally as:
- Doktor kaže da... = The doctor says that...