U supermarketu sam kupila ananas i dvije boce vode.

Because u changes case depending on meaning:

• u + locative = in a place, static location
  
  • u supermarketu = in the supermarket
• u + accusative = into / to a place, movement toward it
  
  • Idem u supermarket = I’m going to the supermarket

So in your sentence, the speaker bought something in the supermarket, so Croatian uses the locative: supermarketu.

Croatian past tense is usually made with two parts:

• a form of biti as an auxiliary: sam = am
• a past participle: kupila = bought

So sam kupila literally works like I have bought, but in normal English we translate it as I bought.

This is the standard way to form the past tense in Croatian.

Both can be correct, but Croatian has a rule that short unstressed words like sam often go in the second position in the clause.

Here, the first element is U supermarketu, so sam comes right after it:

• U supermarketu sam kupila ananas i dvije boce vode.

You can also say:

• Kupila sam ananas i dvije boce vode u supermarketu.

The difference is mostly about word order and emphasis, not basic meaning.

Yes.

In Croatian, the past participle agrees with the subject’s gender and number:

• kupio = a male speaker
• kupila = a female speaker
• kupili = mixed group / masculine plural
• kupile = feminine plural

So sam kupila tells you the speaker is female.

Croatian often leaves out subject pronouns when they are not needed.

In this sentence, sam already shows first person singular, so ja is unnecessary.

• U supermarketu sam kupila... = I bought...
• Ja sam kupila... is also possible, but it sounds more emphatic, like I bought...

This is very common in Croatian.

Because it is the direct object of kupiti, so it is in the accusative case.

But for many masculine inanimate singular nouns, the accusative looks exactly the same as the nominative.

So:

• nominative: ananas
• accusative: ananas

That is why it looks unchanged.

Compare this with a masculine animate noun, where the accusative usually changes:

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

Because Croatian numbers agree in gender.

• dva is used with masculine and neuter nouns
• dvije is used with feminine nouns

Since boca is a feminine noun, you must say:

• dvije boce

Examples:

• dva ananasa
• dva sela
• dvije boce

After 2, 3, and 4, Croatian uses a special counting form of the noun.

So you get:

• jedna boca
• dvije boce
• tri boce
• četiri boce
• pet boca

So after dvije, the correct form is boce.

A useful pattern to remember is:

• 2–4: boce
• 5 and more: boca

Because boce vode means bottles of water, and Croatian usually puts the second noun into the genitive after a container or quantity word.

So:

• boca vode = bottle of water
• dvije boce vode = two bottles of water

This same pattern appears in many expressions:

• čaša vode = glass of water
• šalica kave = cup of coffee
• kilogram jabuka = kilogram of apples

Here, vode is the genitive singular of voda.

Yes, Croatian word order is fairly flexible, but the emphasis changes.

For example:

• U supermarketu sam kupila ananas i dvije boce vode.
  Neutral, with focus on the setting first.

• Kupila sam ananas i dvije boce vode u supermarketu.
  Also natural; starts with the action.

• Ananas i dvije boce vode sam kupila u supermarketu.
  Puts more focus on what was bought.

The meaning stays basically the same, but Croatian uses word order much more for focus and style than English does.