Narodu su potrebni davaoci krvi.

Because with potreban / potrebna / potrebno / potrebni in this kind of sentence, Serbian often marks the person or group who needs something with the dative.

So:

• narodu = to the people / for the people
• davaoci krvi = the thing that is needed

A very literal way to understand the structure is:

• Narodu su potrebni davaoci krvi.
• To the people, blood donors are necessary.

This is a very common Serbian pattern.

The grammatical subject is davaoci krvi.

You can tell because:

• it is in the nominative plural
• the adjective potrebni agrees with it
• the verb su is plural

So the sentence is built like this:

• narodu = indirect object / experiencer in the dative
• su potrebni = are needed
• davaoci krvi = subject, blood donors

Even though in English we often say The people need blood donors, Serbian structures it differently here.

Because potrebni has to agree with davaoci, which is:

• masculine
• plural

Agreement in Serbian works like this:

• potreban = masculine singular
• potrebna = feminine singular
• potrebno = neuter singular
• potrebni = masculine plural

Since davaoci is masculine plural, the correct form is potrebni.

Because su is a clitic, and Serbian clitics usually go in the second position of the sentence or clause.

So in:

• Narodu su potrebni davaoci krvi.

the first unit is Narodu, and then the clitic su comes right after it.

This is very normal Serbian word order.

You may also see other word orders, such as:

• Davaoci krvi su potrebni narodu.
• Potrebni su davaoci krvi narodu.

But in each version, su still tries to stay near the beginning, in the clitic position.

Because davaoci krvi means donors of blood.

In Serbian, when one noun describes another like X of Y, the second noun is often in the genitive.

So:

• krv = blood
• krvi = of blood

Compare:

• čaša vode = a glass of water
• grupa ljudi = a group of people
• davaoci krvi = donors of blood

So krvi is genitive singular here.

Grammatically, narod is singular.

It is a collective noun meaning something like:

• the people
• the nation
• the populace

So:

• nominative: narod
• dative singular: narodu

Even though the meaning refers to many people, the noun itself is singular in grammar.

Yes, that is a very good literal translation for understanding the grammar.

A more natural English translation would usually be:

• The people need blood donors.
• Blood donors are needed.
• Blood donors are needed by the people / for the people.

But for learning the Serbian structure, To the people, blood donors are necessary is very helpful.

Yes. That is also possible, and it means roughly the same thing.

Compare the two patterns:

1. Narodu su potrebni davaoci krvi.

   • literally: Blood donors are needed to the people
   • more descriptive, impersonal, slightly formal

2. Narod treba davaoce krvi.

   • literally: The people need blood donors
   • more direct, with a normal subject + verb + object structure

Notice the cases:

• narod = nominative subject
• treba = needs
• davaoce krvi = accusative plural object

So yes, both are possible, but the grammar is different.

Yes, Serbian word order is flexible.

You could also hear:

• Davaoci krvi su potrebni narodu.
• Narodu su davaoci krvi potrebni.

These versions all keep basically the same meaning, but the emphasis changes.

For example:

• Narodu su potrebni davaoci krvi.
  starts with narodu, so it highlights who needs them
• Davaoci krvi su potrebni narodu.
  starts with davaoci krvi, so it highlights what is needed

The main things that must stay correct are:

• narodu in dative
• davaoci krvi in nominative plural
• potrebni agreeing with davaoci

The singular is davalac or in some usage davaoc, depending on the noun pattern and standard/formal preference.

In this sentence, the plural davaoci means donors.

For learners, the important thing is to recognize:

• davaoci krvi = blood donors

You do not need to worry too much at first if the singular-plural formation looks irregular. Serbian has a number of masculine nouns whose plural forms are not completely predictable from the English point of view.

Because Serbian has no articles like English a / an / the.

So:

• narod can mean people, the people, or a people, depending on context
• davaoci krvi can mean blood donors or the blood donors, depending on context

Serbian relies on context, word order, and the overall situation instead of articles.

Yes, it sounds natural, and it is somewhat formal/public-information style.

It sounds like something you might see in:

• a health campaign
• a public announcement
• a charity appeal
• a hospital message

That is because potrebni often appears in official or public wording:

• Potrebni su volonteri. = Volunteers are needed.
• Potrebna je pomoć. = Help is needed.
• Potrebni su davaoci krvi. = Blood donors are needed.

So the sentence feels very normal in that kind of context.