Ja ću donijeti kruh, a ti kupi ostalo.

That’s true: Croatian usually omits subject pronouns when the verb already shows the person.

So without emphasis, you could say:

• Donijet ću kruh, a kupi ostalo.

Here, though, ja and ti are used for contrast:

• I will bring the bread,
• you buy the rest.

So the pronouns are not necessary for grammar, but they help highlight the division of tasks.

Ću donijeti is the future tense:

• ću = I will
• donijeti = to bring

So:

• ja ću donijeti = I will bring

Croatian future 1 is usually made with:

• a form of htjeti (ću, ćeš, će, ćemo, ćete, će)
• • the infinitive

Examples:

• Ja ću donijeti. = I will bring.
• Ti ćeš kupiti. = You will buy.

Both are possible, just with different word order.

• Ja ću donijeti kruh.
• Donijet ću kruh.

Both mean I will bring bread.

A useful thing to know is:

• when the future auxiliary comes before the infinitive, you use the full infinitive: ću donijeti
• when it comes after the infinitive, Croatian usually writes the infinitive in the shortened form: donijet ću

So this is mainly a matter of word order and standard spelling, not a change in meaning.

Here donijeti means to bring.

A good way to think of it:

• donijeti = bring something to a place / toward the destination
• odnijeti = take something away
• ponijeti = take something along with you

So in this sentence, the speaker is saying they will bring the bread to wherever the speaker and listener are meeting or need it.

Because kupi is the imperative: buy!

So the sentence mixes:

• future for the speaker’s action: Ja ću donijeti kruh
• imperative for the other person’s action: a ti kupi ostalo

This is very natural in Croatian. It sounds like making a plan or giving instructions:

• I’ll bring bread, and you buy the rest.

If you said a ti ćeš kupiti ostalo, that would sound more like a plain statement about what will happen, not a direct instruction or suggestion.

Kupi is the 2nd person singular imperative of kupiti (to buy).

So:

• kupi! = buy! (said to one person)

Compare:

• kupi = singular imperative
• kupite = plural or polite imperative

Examples:

• Kupi kruh. = Buy bread.
• Kupite kruh. = Buy bread. / Please buy bread.

In this sentence, a links two clauses with a slight sense of contrast or division:

• Ja ću donijeti kruh, a ti kupi ostalo.

It is like saying:

• I’ll bring bread, and as for you, buy the rest.

So a often works like:

• and
• while
• whereas
• sometimes almost but

Using i would sound more like simply adding another action.
Using a is better here because the speaker is assigning different roles to two different people.

Here ostalo means the rest or everything else.

It is being used on its own, without a noun after it. In other words, Croatian can use this form like English the rest.

So:

• kupi ostalo = buy the rest
• buy the remaining things

The noun is understood from context, so Croatian does not need to say exactly the rest of the groceries/items/things.

Kruh is the direct object of donijeti, but for many masculine inanimate nouns in Croatian, the accusative singular looks the same as the nominative singular.

So:

• nominative: kruh
• accusative: kruh

That is why you do not see a different ending here.

Compare that with a masculine animate noun, where nominative and accusative are different:

• pas = dog
• vidim psa = I see the dog

But with kruh:

• donosim kruh = I am bringing bread

Because Croatian has no articles.

English distinguishes:

• bread
• a bread
• the bread

Croatian usually just says:

• kruh

The exact meaning depends on context.

So Ja ću donijeti kruh could mean:

• I’ll bring bread
• I’ll bring the bread

In real life, the situation tells you which one is meant.

Yes, Croatian word order is fairly flexible, but not completely free.

This sentence can be rearranged for emphasis, for example:

• Donijet ću kruh, a ti kupi ostalo.
• Kruh ću donijeti ja, a ostalo kupi ti.

These versions shift emphasis, but the basic meaning stays similar.

One important rule is that short words like ću tend to appear in the second position of their clause, so word order changes are possible, but they still follow clitic-placement rules.

Here it is the subject pronoun you.

Croatian ti can also be a dative form meaning to you, but in this sentence it is clearly the subject because:

• it is paired with ja
• it follows a, creating contrast: ja ... a ti ...
• the verb kupi is an imperative addressed to you

So here:

• a ti kupi ostalo = and you buy the rest