Moj brat će doneti tortu, a ja ću doneti sok.

Serbian often forms the future with:

• a short form of hteti (ću, ćeš, će, ćemo, ćete, će)
• plus the infinitive of the main verb

So here:

• će doneti = he will bring
• ću doneti = I will bring

This is a very common and normal way to make the future in Serbian.

You may also see fused forms in Serbian, such as:

• doneću = I will bring
• doneće = he/she will bring

Both patterns are standard, but ću doneti / će doneti is especially useful for learners because it clearly shows the auxiliary and the infinitive separately.

Because the short future auxiliary changes according to the subject.

Here are the relevant forms:

• ja ću = I will
• ti ćeš = you will
• on/ona/ono će = he/she/it will
• mi ćemo = we will
• vi ćete = you (plural/formal) will
• oni/one/ona će = they will

So:

• Moj brat će doneti tortu → My brother will bring the cake
• Ja ću doneti sok → I will bring the juice

Since brat is he, it takes će.
Since ja is I, it takes ću.

Because the short forms like ću and će are clitics in Serbian. Clitics usually appear in the second position of the clause.

So in:

• Moj brat će doneti tortu

the first unit is Moj brat, and the clitic će comes right after that.

Likewise:

• a ja ću doneti sok

the first unit is ja, and ću comes after it.

This “second-position” behavior is very important in Serbian. English speakers often expect freer placement, but these short auxiliary forms have preferred positions.

In this sentence, a links two clauses and shows a mild contrast or a change of topic:

• Moj brat će doneti tortu, a ja ću doneti sok.

It feels like:

• My brother will bring the cake, and I will bring the juice
• or more literally: My brother will bring the cake, while/as for me, I will bring the juice

Why not the others?

• i = simple and
• ali = stronger but
• a = contrastive and / while / whereas / and as for...

So a is the most natural choice when two people are doing different but complementary things.

Because tortu is the accusative case of torta.

The verb doneti (to bring) takes a direct object, and direct objects usually go in the accusative.

So:

• torta = nominative, the basic dictionary form
• tortu = accusative, used as the object

Compare:

• Torta je na stolu. = The cake is on the table.
• Donosim tortu. = I am bringing the cake.

Since the cake is what is being brought, Serbian uses tortu.

Because sok here is also a direct object, but masculine inanimate nouns often have the same form in nominative and accusative singular.

So:

• sok = nominative singular
• sok = accusative singular as well

That is why the form does not change.

Compare:

• Sok je hladan. = The juice is cold.
• Donosim sok. = I am bringing the juice.

By contrast, feminine nouns like torta usually do change:

• torta → tortu

Yes, you could say Brat će doneti tortu, but it would usually sound less natural unless the context already makes it clear whose brother you mean.

Moj brat means my brother, and Serbian often uses possessives just like English does when that relationship matters.

So:

• Moj brat = my brother
• Brat = brother

If you just say brat, it can sound more general or context-dependent. In this sentence, moj helps identify the person clearly.

Yes, Serbian often drops subject pronouns because the verb form already shows the person.

For example, ću doneti sok already means I will bring juice.

But ja is included here for emphasis or contrast:

• Moj brat će doneti tortu, a ja ću doneti sok.

This is like saying:

• My brother will bring the cake, and I will bring the juice.

The pronoun ja helps highlight the contrast between my brother and me. So although it is not strictly necessary for grammar, it is very natural here.

Doneti is a perfective verb. It refers to a completed action: to bring in the sense of successfully bringing something.

That makes it very natural in this future sentence, because the idea is that each person will complete the action:

• my brother will bring the cake
• I will bring the juice

Its imperfective partner is usually donositi:

• doneti = perfective, completed bringing
• donositi = imperfective, repeated/ongoing bringing

For example:

• Sutra ću doneti tortu. = Tomorrow I’ll bring the cake.
• On često donosi tortu. = He often brings cake.

They are related, but they are different forms.

• doneti = infinitive, to bring
• donesi = imperative, bring!
• doneću = future, I will bring

So in your sentence:

• ću doneti = I will bring

The infinitive itself is doneti.

Yes, that is also possible in Serbian.

You may see both:

• Moj brat će doneti tortu, a ja ću doneti sok.
• Moj brat doneće tortu, a ja doneću sok.

The meaning is the same.

The first version keeps the auxiliary separate:

• ću/će + infinitive

The second version uses the attached future form:

• doneću / doneće

Both are standard, though learners often find the separate form easier to analyze at first.

They are pronounced roughly like this:

• ću ≈ chyoo
• će ≈ chyeh

More exactly, Serbian ć is a soft sound that does not match English perfectly. It is softer than English ch.

Also note:

• ću has the letter u
• će has the letter e

So they are different words and different forms.

Because the sentence has two coordinated clauses:

• Moj brat će doneti tortu
• a ja ću doneti sok

In Serbian, it is normal to separate such clauses with a comma, especially when a connects them. The comma helps show the division and the contrast between the two parts.

So the punctuation here is standard and natural.