U supermarketu sam kupila ananas i dvije mandarine.

Because u can mean either in or into, and the case depends on whether there is location or movement.

• u supermarketu = in the supermarket → location, so Croatian uses the locative
• u supermarket = would suggest motion into the supermarket, which would normally require the accusative, but with this noun you would usually still hear a fully declined form in a complete sentence such as idem u supermarket

In your sentence, the person is saying where the buying happened, so u supermarketu means in the supermarket.

Because it is in the locative singular after u when talking about location.

The base noun is:

• supermarket = supermarket

Locative singular:

• u supermarketu = in the supermarket

This -u ending is very common for masculine nouns in the locative singular.

Because Croatian forms the past tense with:

• a form of biti (to be)
• plus the past participle

So:

• sam kupila = I bought

Here:

• sam = I am / have-type auxiliary used in the past tense
• kupila = past participle of kupiti (to buy)

Together they make the past tense: I bought.

Croatian usually does not use a single-word simple past the way English does.

This is because sam is a clitic in Croatian. Clitics are short unstressed words that usually want to appear in the second position of the sentence or clause.

So instead of:

• U supermarketu kupila sam ananas...

you often get:

• U supermarketu sam kupila ananas...

Both can be understood, but the version with sam early in the sentence is very natural because Croatian clitics tend to come near the beginning.

Because the speaker is female.

In Croatian past tense, the participle agrees with the gender of the subject:

• kupio = a man says I bought
• kupila = a woman says I bought
• kupilo = neuter, used in other contexts

So:

• sam kupila = I bought, said by a woman

If a man were speaking, the sentence would be:

• U supermarketu sam kupio ananas i dvije mandarine.

Because Croatian often drops subject pronouns when they are clear from the verb form.

Here, sam kupila already tells you:

• first person singular = I
• feminine speaker = female I

So ja (I) is unnecessary unless the speaker wants emphasis.

Compare:

• Sam kupila ananas. / Kupila sam ananas. = I bought a pineapple.
• Ja sam kupila ananas. = I bought a pineapple. (emphatic)

Because ananas here is the direct object of kupila, so it is in the accusative.

For many masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: ananas
• accusative: ananas

That is why the form does not change in this sentence.

Because mandarina is a feminine noun, and the number two agrees in gender.

• dva = two, for masculine/neuter
• dvije = two, for feminine

So:

• dvije mandarine = two mandarins

Examples:

• dva ananasa = two pineapples
• dvije mandarine = two mandarins

Because after dvije, the noun takes a special counting form that looks like the genitive singular for many feminine nouns.

So:

• mandarina = mandarin, nominative singular
• dvije mandarine = two mandarins

This is normal with numbers such as dva/dvije, tri, and četiri.

For a learner, the easiest practical rule is:

• after dvije, feminine nouns often appear in -e

Because Croatian does not have articles like English a/an/the.

So:

• ananas can mean a pineapple or the pineapple
• dvije mandarine can mean two mandarins or the two mandarins, depending on context

The exact meaning is usually understood from the situation.

Croatian word order is fairly flexible, though some versions sound more natural than others depending on emphasis.

Your sentence:

• U supermarketu sam kupila ananas i dvije mandarine.

Possible alternatives include:

• Kupila sam ananas i dvije mandarine u supermarketu.
• Ananas i dvije mandarine sam kupila u supermarketu.

These all mean roughly the same thing, but the focus changes:

• U supermarketu... emphasizes where
• Kupila sam... is more neutral
• Ananas i dvije mandarine... emphasizes what was bought

So the original sentence is natural, especially if the speaker wants to highlight the location first.

I means and.

So:

• ananas i dvije mandarine = a pineapple and two mandarins

It simply joins the two things that were bought.

A man would say:

• U supermarketu sam kupio ananas i dvije mandarine.

The only change is:

• kupila → kupio

Everything else stays the same.