U košarici su krafna, burek i još jedno toplo pecivo.

Because u means in here in the sense of location, and after u Croatian normally uses the locative case for a static location.

• dictionary form: košarica = basket
• locative singular: košarici = in the basket

So:

• u košarici = in the basket

A very useful contrast is:

• u košarici = in the basket, inside it
• u košaricu = into the basket, movement toward it

So the sentence uses košarici because it describes where the items are, not motion.

Because the subject is not just one noun. It is a list of three things:

• krafna
• burek
• još jedno toplo pecivo

Together, they form a compound subject, and Croatian uses a plural verb with that.

So:

• krafna je = the doughnut is
• krafna, burek i pecivo su = the doughnut, the burek, and the pastry are

Even though each item is singular by itself, the whole set is plural.

This is normal Croatian word order.

There are two things going on:

1. U košarici is placed first because the sentence starts with the location.
2. su is a short unstressed form, and these short forms usually go in second position in the sentence or after the first phrase.

So:

• U košarici su krafna, burek i još jedno toplo pecivo.

is very natural.

You could also say:

• Krafna, burek i još jedno toplo pecivo su u košarici.

That version is also correct, but it puts more emphasis on the items themselves rather than on where they are.

Because they are the things being identified as the ones that are in the basket. In this structure, Croatian uses the nominative.

So you are basically saying:

• In the basket are X, Y, and Z.

That is why you get:

• krafna
• burek
• pecivo

and not accusative forms.

This can feel strange to an English speaker because English uses there is / there are without visible case changes. In Croatian, the nouns here are not direct objects.

Because Croatian does not have articles.

So where English says:

• a doughnut
• the basket

Croatian simply says:

• krafna
• košarica / košarici

Whether something is understood as a, the, or just a general noun depends on context, word order, and sometimes other words such as demonstratives:

• ta krafna = that doughnut
• ova košarica = this basket

So in this sentence, the English translation may need articles, but Croatian does not.

Još jedno means one more or another.

Literally, još often means still, yet, more, and jedno means one in the neuter form.

So:

• još jedno pecivo = one more pastry / another pastry

It suggests an additional item beyond the ones already mentioned.

Because both jedno and toplo must agree with pecivo in gender, number, and case.

Here:

• pecivo is neuter singular
• so the words that describe it must also be neuter singular

That gives:

• jedno toplo pecivo

Compare the three genders:

• jedna topla krafna = one warm doughnut
• jedan topli burek = one warm burek
• jedno toplo pecivo = one warm pastry

This sentence is actually a nice example of all three genders:

• krafna = feminine
• burek = masculine
• pecivo = neuter

Sometimes yes, but it changes the structure and often the feel of the sentence.

U košarici su ... is good when you are naming the specific things that are in the basket.

With ima, Croatian often sounds more existential or indefinite, and the noun phrase is commonly different in form.

For example:

• U košarici ima peciva. = There is pastry / there are pastries in the basket.
• U košarici su krafna, burek i još jedno toplo pecivo. = In the basket are a doughnut, a burek, and one more warm pastry.

So for a clear list of specific items, su is very natural here.

Yes. Croatian word order is fairly flexible.

Possible versions include:

• U košarici su krafna, burek i još jedno toplo pecivo.
• Krafna, burek i još jedno toplo pecivo su u košarici.

Both are grammatical, but they do not sound exactly the same in focus.

• Starting with U košarici emphasizes the location.
• Starting with the food items emphasizes what the items are.

The original version is very natural if the basket is already the topic.

Because in a simple list, Croatian normally does not put a comma before i.

So this is standard:

• krafna, burek i još jedno toplo pecivo

This is similar to English without the Oxford comma:

• doughnut, burek and one more warm pastry

In Croatian, the no-comma version before i is the normal choice in an ordinary list.

The dictionary forms are:

• košarica = basket
• krafna = doughnut
• burek = burek
• pecivo = pastry / baked good

In the sentence:

• košarici is a changed form of košarica because of the locative case
• krafna, burek, and pecivo are already in their base nominative singular forms

This is useful because when you look words up in a dictionary, you would search for:

• košarica
• krafna
• burek
• pecivo