Konobarica je donijela kavu i kolače na velikom poslužavniku.

Because this is the usual Croatian past tense formation.

• je = the auxiliary verb biti (to be) in the 3rd person singular
• donijela = the past participle of donijeti (to bring)

So je donijela literally works like has brought / brought.

In standard Croatian, past tense is often made with:

• auxiliary sam, si, je, smo, ste, su
• plus a past participle that agrees with the subject in gender and number

Here the subject is konobarica (waitress), which is feminine singular, so we get donijela.

Because the participle has to agree with the subject.

The subject is konobarica, which is:

• feminine
• singular

So the correct participle is:

• masculine singular: donio
• feminine singular: donijela
• neuter singular: donijelo

Examples:

• Konobar je donio kavu. = The waiter brought the coffee.
• Konobarica je donijela kavu. = The waitress brought the coffee.

Konobarica is in the nominative singular because it is the subject of the sentence.

The subject is the person doing the action:

• Konobarica je donijela... = The waitress brought...

If it were the object, the form would be different in some contexts, but here it is simply the subject.

Because kavu is the accusative singular of kava.

The verb donijeti takes a direct object, and direct objects are usually in the accusative.

So:

• nominative: kava = coffee
• accusative: kavu = coffee (as the thing being brought)

In this sentence, the waitress brought the coffee, so kava changes to kavu.

Because kolače is the accusative plural of kolači / kolač.

Again, this is a direct object after donijela, so it goes into the accusative.

For this noun:

• nominative plural: kolači = cakes
• accusative plural: kolače = cakes (as objects)

So:

• kolači su na stolu = the cakes are on the table
• donijela je kolače = she brought the cakes

Because both are direct objects of the same verb donijela.

She brought:

• kavu
• i
• kolače

Croatian often works like English here: if two things are both objects of the same verb, they both appear in the required case.

So the structure is:

• donijela [kavu] i [kolače]

Both noun phrases answer the question What did she bring?

Because na can mean either:

• onto something, with the accusative
• on something, with the locative

Here the meaning is on a large tray, describing where the coffee and cakes were, not movement onto the tray at that moment. So Croatian uses:

• na + locative → na velikom poslužavniku

Compare:

• Stavila je šalicu na stol. = She put the cup onto the table. (accusative)
• Šalica je na stolu. = The cup is on the table. (locative)

In your sentence, the tray is the surface the items are on, so na + locative is natural.

Because the adjective and noun must agree in:

• case
• number
• gender

Here:

• poslužavnik is masculine singular
• it comes after na, which here requires the locative
• so the adjective must also be masculine singular locative

That gives:

• velik → velikom
• poslužavnik → poslužavniku

So na velikom poslužavniku means on a large tray, with full agreement inside the phrase.

It is locative singular.

The dictionary form is:

• poslužavnik = tray

In the sentence:

• na poslužavniku = on the tray

Because na here expresses location, it takes the locative, so:

• poslužavnik → poslužavniku

Yes, poslužavnik is a normal word for tray.
Yes, pladanj is also a common word and often means the same thing.

Depending on speaker and region, one may sound more usual than the other. In many everyday situations, both are understandable.

So you may hear:

• na poslužavniku
• na pladnju

Both can mean on a tray.

Croatian auxiliaries like je are clitics, and clitics usually go in the second position of the sentence or clause.

So the neutral order is:

• Konobarica je donijela kavu i kolače...

This is very typical Croatian word order.

You will often see similar patterns:

• On je došao.
• Marija je kupila kruh.
• Jučer je padala kiša.

Notice that je usually comes after the first element:

• Jučer je...
• Konobarica je...

It is not simply “the second word” in a strict English-like sense, but learners can often start by thinking of it as a second-position clitic.

Yes. Croatian word order is fairly flexible, and different orders can change the focus or emphasis.

Neutral:

• Konobarica je donijela kavu i kolače na velikom poslužavniku.

Other possible orders:

• Kavu i kolače konobarica je donijela na velikom poslužavniku.
• Na velikom poslužavniku konobarica je donijela kavu i kolače.

These alternatives are grammatical, but they sound more marked and usually emphasize a particular part of the sentence.

For learners, the original version is a very natural neutral sentence.

This is an important difference.

• donijeti = to bring, usually as a completed action
• nositi = to carry / be carrying / wear, often more ongoing or general

In your sentence:

• je donijela means she brought the coffee and cakes, and the action is complete

If you said something with nositi, it would suggest the action of carrying rather than the completed arrival:

• Konobarica nosi kavu. = The waitress is carrying / bringing coffee.

So donijeti is a good choice when the food or drink has been successfully brought to the table.

Yes, donijeti is a perfective verb.

That matters because Croatian aspect shows whether an action is viewed as:

• completed / whole → perfective
• ongoing / repeated / habitual → imperfective

Here:

• donijeti = to bring, as a completed event
• imperfective partner: often donositi = to be bringing / to bring repeatedly

Compare:

• Konobarica je donosila kavu. = The waitress was bringing coffee / used to bring coffee.
• Konobarica je donijela kavu. = The waitress brought the coffee.

So the sentence presents one completed action.

Because Croatian has no definite or indefinite articles like English the and a/an.

So:

• konobarica can mean a waitress or the waitress
• kavu can mean coffee or the coffee
• kolače can mean cakes or the cakes

The exact meaning depends on context.

In many translations, English has to choose a or the, but Croatian does not need to.

It most naturally means a coffee / a cup of coffee in this context.

Even though kava is literally coffee, in restaurant and café situations Croatian often uses it the same way English does:

• kava = coffee
• but in context, often a coffee as an ordered drink

So donijela kavu i kolače is naturally understood as bringing a serving of coffee along with cakes.

Not if you want the same meaning.

• na velikom poslužavniku = on a large tray
• s/sa velikim poslužavnikom = with a large tray

These are different ideas.

Your sentence means the coffee and cakes were placed on the tray.
If you said sa velikim poslužavnikom, the focus would be that the waitress came with a large tray, which is a different structure and slightly different meaning.

A learner often notices the nije / nije / nje type sequence here.

donijela is pronounced roughly like:

• do-NYE-la

The ni + je here does not sound like English nee-yeh as two fully separate syllables in careful learner pronunciation. In normal speech it flows smoothly.

A simple learner-friendly breakdown is:

• do | ni | je | la but in natural pronunciation it sounds more compact:
• do-NYE-la

The stress pattern can vary by description system, but for a beginner the important thing is to say it smoothly, not letter by letter.

Then the object would become singular.

You would say:

• Konobarica je donijela kavu i kolač na velikom poslužavniku.

Why?

• kolač is masculine inanimate singular
• the accusative singular of masculine inanimate nouns is the same as the nominative singular
• so kolač stays kolač

Compare:

• singular: kolač
• plural accusative: kolače

So the plural form in your sentence tells us there were multiple cakes or pastries.