Davalac krvi čeka izvan ambulante.

Because krvi is the genitive singular form of krv. In Serbian, after a noun like davalac to show what someone gives or donates, the second noun is often put in the genitive.

So:

• krv = nominative, basic dictionary form
• krvi = genitive singular

That is why Serbian says davalac krvi, literally something like giver/donor of blood.

Because the preposition izvan requires the genitive case. The noun ambulanta therefore changes to its genitive singular form ambulante.

So:

• ambulanta = nominative
• ambulante = genitive singular

This is similar to other Serbian prepositions that force the genitive, such as pored, kod, blizu, and ispred.

Usually no. This is a common false friend for English speakers.

In Serbian, ambulanta normally means a clinic, medical office, outpatient department, or similar medical facility.

If you mean the vehicle ambulance, Serbian more often uses expressions like:

• kola hitne pomoći
• sanitetsko vozilo

So in this sentence, ambulanta is a place, not a vehicle.

Davalac means giver or donor. It is formed from the verb davati, meaning to give.

In medical or official language, davalac krvi is the standard phrase for blood donor.

You may also see:

• dobrovoljni davalac krvi = voluntary blood donor

So davalac is not just any random giver here; in this context it specifically means a donor.

Because Serbian does not have articles like English a/an and the.

Whether something is definite or indefinite is usually understood from:

• context
• word order
• intonation
• additional words such as demonstratives

So Davalac krvi can mean a blood donor or the blood donor, depending on context.

Čeka is the present tense, third person singular, of the verb čekati.

It can correspond to either:

• waits
• is waiting

in English, depending on context.

In this sentence, the most natural English meaning is usually is waiting, because it sounds like an action happening right now.

Yes. Just like English wait, Serbian čekati can be used without an object if the object is unknown, obvious, or unimportant.

So:

• Davalac krvi čeka. = The blood donor is waiting.

If you want to say what the person is waiting for, you can add it:

• Davalac krvi čeka doktora. = The blood donor is waiting for the doctor.

No. Serbian word order is fairly flexible.

Davalac krvi čeka izvan ambulante. is a neutral, natural order.

But other orders are possible, for example:

• Izvan ambulante čeka davalac krvi.
  This puts more focus on outside the clinic.

• Čeka davalac krvi izvan ambulante.
  This is less neutral and would depend more on context or emphasis.

So the original sentence is a straightforward, unmarked way to say it.

Yes. Van ambulante is also correct.

Both izvan and van can mean outside. In many contexts they are very close in meaning.

A rough difference is:

• izvan can sound a bit more formal or precise
• van is very common in everyday speech

Both are followed by the genitive, so you still say ambulante, not ambulanta.

Because Serbian usually expresses that kind of relationship through case, not through a separate word like English of.

So instead of saying donor of blood with a separate preposition, Serbian simply uses the genitive:

• davalac krvi

This is very common in Serbian. Compare:

• čaša vode = a glass of water
• boja zida = the color of the wall
• donor organa = organ donor

So the idea of of blood is built into the form krvi.