Za transfuziju je potrebna jedinica krvi.

Because za usually takes the accusative case when it means for.

The base noun is transfuzija (transfusion), which is a feminine noun ending in -a. In the accusative singular, that -a changes to -u:

• transfuzija = nominative
• transfuziju = accusative

So za transfuziju means for a transfusion.

Because potrebna agrees with jedinica, which is a feminine singular noun.

In Serbian, adjectives must agree with the noun they describe in gender, number, and case.

Here:

• jedinica = feminine singular
• so the adjective is potrebna = feminine singular

Compare:

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

So the structure is basically:

• jedinica krvi je potrebna = a unit of blood is needed

Because the subject is jedinica krvi (a unit of blood), and that is grammatically singular.

Even though blood may feel like a substance rather than a countable item in English, the Serbian subject here is jedinica (unit), which is one unit, so the verb is singular:

• je = is

If there were more than one unit, you would need a plural form.

Because after a noun like jedinica (unit), Serbian normally uses the second noun in the genitive case to mean a unit of something.

So:

• jedinica = unit
• krv = blood
• krvi = of blood

That gives:

• jedinica krvi = a unit of blood

This is very common in Serbian:

• čaša vode = a glass of water
• flaša vina = a bottle of wine
• komad hleba = a piece of bread

Because it is in the genitive singular.

The dictionary form is:

• krv = blood

But after jedinica it becomes:

• krvi = of blood

So jedinica krvi literally means unit of blood.

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

So jedinica krvi can mean:

• a unit of blood
• the unit of blood

Which one is meant depends on context. In this sentence, English would normally translate it as a unit of blood.

The sentence is:

• Za transfuziju = for the transfusion / for a transfusion
• je potrebna = is needed
• jedinica krvi = a unit of blood

A very natural English-like rearrangement would be:

• Jedinica krvi je potrebna za transfuziju.

That may make the grammar easier to see, because the subject comes first.

The original order puts za transfuziju at the front for emphasis or context, something like:

• For the transfusion, a unit of blood is needed.

Serbian word order is more flexible than English word order.

Yes, that is also possible.

Serbian allows several word orders because case endings show the grammatical relationships. These all sound natural, with slightly different emphasis:

• Za transfuziju je potrebna jedinica krvi.
• Jedinica krvi je potrebna za transfuziju.
• Potrebna je jedinica krvi za transfuziju.

The meaning stays essentially the same, but the focus shifts a little depending on what comes first.

Because potrebno is neuter singular, and here the adjective should agree with jedinica, which is feminine.

So:

• jedinica → potrebna

You may see potrebno je in impersonal sentences, for example:

• Potrebno je da dođete. = It is necessary that you come.
• Potrebno je više krvi. = More blood is needed.

But in your sentence, the speaker uses a clear noun subject:

• jedinica krvi je potrebna

So agreement with jedinica is required.

Yes. Jedna jedinica krvi means one unit of blood and is more explicit.

Compare:

• jedinica krvi = a unit of blood
• jedna jedinica krvi = one unit of blood

Serbian often leaves out jedna when English would use a/an. Since Serbian has no article, the bare noun often covers that meaning.

It can correspond to either in English, depending on context.

Because Serbian has no articles, transfuziju by itself does not tell you whether English should use:

• for transfusion
• for a transfusion
• for the transfusion

In most ordinary translation contexts, for a transfusion is the most natural English rendering.

It means needed, necessary, or required, depending on context.

In this sentence, potrebna is best understood as:

• needed
• or required

So the sentence means something like:

• A unit of blood is needed for a transfusion.

A rough pronunciation guide is:

Za transfuziju je potrebna jedinica krvi
zah trahn-sfoo-ZEE-yoo yeh poh-TREB-nah yeh-DEE-nee-tsah KRVEE

A few useful notes:

• j is pronounced like English y
• c is pronounced like ts
• ž would sound like the s in measure, but it does not appear here
• krv is a short, compact syllable; Serbian allows consonant-heavy words like this

It is an international medical word, very similar to English transfusion. Serbian uses many such learned or medical terms, often adapted to Serbian spelling and endings.

So:

• English: transfusion
• Serbian: transfuzija

This is why it may look familiar, even though its Serbian grammar behaves like a regular feminine noun ending in -a.