Bundevu i kukuruz kupila sam na tržnici, jer su danas bili jeftiniji nego u supermarketu.

Because bundevu is the accusative singular form of bundeva. In this sentence, it is a direct object of kupiti (to buy), so Croatian uses the accusative case.

• nominative: bundeva
• accusative: bundevu

This is very common with feminine nouns ending in -a.

Because kukuruz is a masculine inanimate noun. In Croatian, masculine inanimate nouns usually have the same form in nominative and accusative singular.

So:

• nominative: kukuruz
• accusative: kukuruz

That is why one object changes (bundevu) and the other does not (kukuruz).

Croatian past tense is normally made with two parts:

• the past participle: kupila
• the auxiliary to be: sam

So kupila sam literally consists of the equivalent of bought + am, but together it simply means I bought.

The form kupila shows that the speaker is feminine. A masculine speaker would say kupio sam.

Croatian word order is much more flexible than English word order. The sentence puts bundevu i kukuruz first for emphasis, as if highlighting what was bought.

A more neutral version would be:

Kupila sam bundevu i kukuruz na tržnici...

The given order is still natural, but it sounds a bit more focused or expressive. Croatian often moves the most important or contrastive information toward the beginning.

Yes, that is also possible. In fact, many learners will often first meet the pattern where the clitic sam appears earlier:

Bundevu i kukuruz sam kupila na tržnici.

That version is also very natural. The given Bundevu i kukuruz kupila sam... is a stylistic word-order choice that keeps kupila sam together and gives the sentence a slightly more marked rhythm.

This is mostly a matter of Croatian preposition usage.

• na tržnici = at the market
• u supermarketu = in the supermarket

With tržnica, Croatian normally uses na, not u, because that is the idiomatic expression. With supermarket, Croatian uses u, since it is understood as being inside a building/store.

So this is not just literal logic; it is also something you learn as a fixed usage pattern.

Both are in the locative singular, because they follow prepositions that express location:

• na tržnici
• u supermarketu

Their base forms are:

• tržnica
• supermarket

So:

• tržnica → tržnici
• supermarket → supermarketu

Because the hidden subject is they, referring to bundevu i kukuruz together. Since there are two things, Croatian uses the plural.

It is masculine plural because when Croatian refers to a mixed group of nouns of different genders, masculine plural is the default agreement form.

That is why you get:

• su bili = they were
• jeftiniji = cheaper in masculine plural

If both nouns were feminine, you would expect bile jeftinije.

Jeftiniji is the comparative form of jeftin.

• jeftin = cheap
• jeftiniji = cheaper

Croatian often forms comparatives with endings like -ji or -iji, though the exact pattern depends on the adjective.

Here jeftiniji also agrees with the understood plural subject, so it is in the masculine plural nominative form.

Because the comparison is really about the price of those items in one place versus another place.

Jeftiniji nego u supermarketu means they were cheaper than in the supermarket.

If you said jeftiniji od supermarketa, that would sound like you were comparing the items to the supermarket itself, which is not the intended meaning. With this kind of location-based comparison, nego + place phrase is very natural.

Because danas means today, but the action and the reason are still being described as part of a completed past event. The speaker bought the items earlier today, and at that time they were cheaper.

So Croatian keeps the narrative in the past:

• kupila sam = I bought
• bili jeftiniji = they were cheaper

This is completely normal.

Because jer introduces a clause giving the reason: because... In standard Croatian, a clause introduced by jer is normally separated from the main clause with a comma.

So the structure is:

• main clause: Bundevu i kukuruz kupila sam na tržnici
• reason clause: jer su danas bili jeftiniji nego u supermarketu

Because Croatian does not have articles like English a/an and the. Whether something is definite or indefinite is usually understood from context.

If Croatian speakers want to be more specific, they can use demonstratives such as ovaj, taj, or onaj, but most of the time no article is needed.