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...

In this sentence, se is not really translated as itself. It is a reflexive particle that belongs with the verb širiti se, which means to spread.

Compare:

• širiti nešto = to spread something
• širiti se = to spread

So:

• Virus se širi = The virus is spreading

English does not usually show this reflexive idea, but Croatian often does. So you should learn širiti se as a verb unit.

Širi is the 3rd person singular present tense of širiti se.

Because the subject is virus (singular), the verb is singular too:

• ja se širim = I spread / am spreading
• ti se širiš = you spread / are spreading
• on/ona/ono se širi = he/she/it spreads / is spreading

So virus se širi means the virus spreads or is spreading, depending on context.

It can mean both.

Croatian present tense often covers both:

• a general present: spreads
• an ongoing present: is spreading

So Virus se brzo širi could mean:

• The virus spreads quickly
• The virus is spreading quickly

In this sentence, because it sounds like a current situation, English would often use is spreading quickly.

Brzo is an adverb meaning quickly. In Croatian, adverbs often come before the verb, especially in neutral word order:

• virus se brzo širi

This is very natural and standard.

You may also hear different word orders for emphasis, such as:

• Virus se širi brzo
• Brzo se virus širi

But Virus se brzo širi is the most neutral and common here.

Because u školi means in the school, indicating location, not movement.

Croatian uses:

• u + locative for being in a place
• u + accusative for movement into a place

So:

• u školi = in the school
• u školu = into the school

In this sentence, the virus is spreading in the school, so Croatian uses the locative:

• škola → u školi

It is the locative singular of škola.

The base form is:

• škola = school

After u when it means in, Croatian uses the locative:

• u školi = in the school

This is a very common pattern:

• u gradu = in the city
• u kući = in the house
• u školi = in the school

Croatian does not have articles like English a and the.

So doktor can mean:

• a doctor
• the doctor

and virus can mean:

• a virus
• the virus

The exact meaning depends on context. In this sentence, English naturally uses the doctor and the virus, but Croatian does not need separate words for that.

Yes, doktor is a normal and very common word. It can refer to:

• a medical doctor
• someone with a doctorate, depending on context

In everyday medical contexts, doktor is very common. You may also see liječnik, which is a more specifically physician/medical doctor word.

So in this sentence, Doktor kaže... is perfectly natural.

Not naturally in this sentence, because virus is the important noun being talked about.

Croatian often drops subject pronouns like ja, ti, on, because the verb ending already shows the person. But nouns are not dropped unless the context makes them completely obvious.

So:

• Doktor kaže da se virus brzo širi u školi. = normal
• Doktor kaže da se brzo širi u školi. = possible only if it is already very clear what it is

Without virus, the sentence becomes less clear.

Croatian word order is fairly flexible, but different orders can sound more marked or emphasize different parts.

The neutral version is:

• Doktor kaže da se virus brzo širi u školi.

Possible alternatives include:

• Doktor kaže da se brzo širi virus u školi.
• Doktor kaže da se virus u školi brzo širi.

These are grammatically possible, but the original sounds the most neutral and natural for a learner to use.

A rough guide:

• š sounds like sh in ship
• č sounds like ch in chocolate
• ž sounds like the s in measure

So:

• širi sounds roughly like shee-ree
• školi sounds roughly like shko-lee

Also, Croatian spelling is quite phonetic, so words are usually pronounced much as they are written.