Lekar pacijentu kaže da diše.

Because pacijentu is the dative singular form of pacijent.

In this sentence, the patient is the person receiving the doctor’s words, so Serbian uses the dative case:

• pacijent = patient
• pacijentu = to the patient

English uses the preposition to, but Serbian usually shows that meaning with a case ending instead.

So:

• Lekar pacijentu kaže... = The doctor tells the patient...

Lekar is in the nominative singular because it is the subject of the sentence, the one doing the action.

Here is the basic structure:

• Lekar = subject, nominative
• pacijentu = indirect object, dative
• kaže = verb
• da diše = subordinate clause

So the sentence is literally structured like:

Doctor to-patient says that breathes

It can correspond to either says or tells, depending on the structure.

In this sentence, because there is a person in the dative case, pacijentu, the most natural English translation is tells:

• Lekar pacijentu kaže... = The doctor tells the patient...

Without that indirect object, kaže is often closer to says:

• Lekar kaže da... = The doctor says that...

So Serbian kaže covers ground that English often splits between say and tell.

Because Serbian very often uses da + present tense where English uses an infinitive.

So:

• English: tells the patient to breathe
• Serbian: kaže pacijentu da diše

This pattern is extremely common in Serbian:

• Hoću da idem = I want to go
• Moram da radim = I have to work
• Kaže da diše = tells him/her to breathe

So even though diše is a present-tense form, the whole da + present construction often functions like an English infinitive.

Yes. Diše is the 3rd person singular present form.

Its dictionary form is disati, meaning to breathe.

So:

• disati = to breathe
• dišem = I breathe
• dišeš = you breathe
• diše = he/she/it breathes

A learner may notice that the stem changes from dis- to diš- in the present tense. That is normal here.

Mostly from context and common sense.

In a sentence like this, the natural interpretation is:

• the doctor is speaking
• the patient is the person being addressed
• da diše is understood as the instruction given to the patient

So the usual meaning is The doctor tells the patient to breathe.

However, grammatically, the subject of diše is not explicitly stated, so the sentence can be somewhat ambiguous in isolation.

If you want to make it clearer, Serbian could use:

• Lekar kaže pacijentu: Dišite. = The doctor tells the patient: Breathe.
• Lekar kaže da pacijent diše. = The doctor says that the patient is breathing.

So the original sentence is normally understood as an instruction, but context is what settles it.

Yes, that is the most natural way to understand it.

Serbian often uses da + present to report commands, requests, or instructions indirectly.

So instead of giving the direct command:

• Diši = Breathe (singular informal)
• Dišite = Breathe (plural or polite singular)

you can report it indirectly:

• Lekar pacijentu kaže da diše.

That is why the Serbian form looks like that he/she breathes on the surface, but in meaning it works like to breathe or breathe.

No, not in this sentence.

In Serbian, when da introduces a clause that functions as the object or complement of a verb like kaže, you normally do not put a comma before it.

So this is correct:

• Lekar pacijentu kaže da diše.

A comma would generally be wrong here.

Yes. Serbian word order is relatively flexible.

These are all possible, with slightly different emphasis:

• Lekar pacijentu kaže da diše.
• Lekar kaže pacijentu da diše.
• Pacijentu lekar kaže da diše.

The meaning stays basically the same, but the focus can shift a little depending on what you want to emphasize.

The given order is perfectly natural.

Because Serbian has no articles.

So:

• lekar can mean a doctor or the doctor
• pacijentu can mean to a patient or to the patient

Context tells you which meaning is intended.

That is why one Serbian sentence can correspond to several English versions:

• The doctor tells the patient to breathe
• A doctor tells the patient to breathe
• The doctor tells a patient to breathe

Yes, in most everyday situations you can.

Both words can mean doctor in the medical sense:

• lekar
• doktor

A small difference is that doktor can also mean someone with a doctorate, depending on context, while lekar is specifically the medical word.

So in a medical sentence, both are usually fine:

• Doktor pacijentu kaže da diše.
• Lekar pacijentu kaže da diše.

Yes. In real life, a doctor would very often use a direct imperative instead of this reported structure.

For example:

• Dišite. = Breathe. / Keep breathing.
• Udahnite. = Inhale.
• Dišite duboko. = Breathe deeply.

So Lekar pacijentu kaže da diše sounds like a description of what is happening, while Dišite is what the doctor would actually say to the patient.