Danas u supermarketu uzimam košaricu, a moj brat uzima kolica.

Because u can take different cases depending on meaning.

• u + locative = in / at a place, when there is no movement into it
• u + accusative = into a place, when there is movement toward it

Here, the sentence describes where the action happens: u supermarketu = in the supermarket.
So supermarket goes into the locative singular: supermarketu.

Compare:

• U supermarketu uzimam košaricu. = I take a basket in the supermarket.
• Idem u supermarket. = I’m going to/into the supermarket.

Because košaricu is the accusative singular form of košarica.

The verb uzimati / uzeti means to take, and the thing being taken is the direct object, which usually goes in the accusative.

So:

• nominative: košarica = basket
• accusative: košaricu = basket, as the object of the verb

In the sentence:

• uzimam košaricu = I am taking a basket

This is a very common pattern in Croatian: the object changes form.

Because kolica is a plural-only noun in Croatian. It is one of those nouns that are grammatically plural even when they refer to a single item.

In this sentence, kolica means a shopping cart / trolley. Grammatically, it behaves like a plural noun.

So you get:

• nominative: kolica
• accusative: kolica

That is why the form does not change here.

This is similar to how some English nouns are plural in form, though the exact usage is not the same. In Croatian, this is a normal feature of certain nouns.

They refer to two different things you use while shopping:

• košarica = a shopping basket, the thing you carry in your hand
• kolica = a shopping cart / trolley, the larger one on wheels

So the sentence contrasts two different choices:

• I take a basket
• my brother takes a cart

A often connects two clauses with a mild contrast or comparison. Here it is like:

• while
• whereas
• sometimes simply and, but with contrast

So:

• Danas u supermarketu uzimam košaricu, a moj brat uzima kolica.

means something like:

• Today in the supermarket, I take a basket, while my brother takes a cart.

If you used i, it would sound more like simple addition:

• ... i moj brat uzima kolica = ... and my brother takes a cart

That is possible in some contexts, but a is more natural here because the two people are being compared.

Because Croatian often omits subject pronouns when they are not needed.

The verb ending already shows who is doing the action:

• uzimam = I take / I am taking
• uzima = he/she takes / is taking

So ja is not necessary here. You could say ja uzimam, but it usually adds emphasis, like:

• Ja uzimam košaricu, a moj brat uzima kolica.
  = I’m the one taking a basket, and my brother is taking a cart.

In ordinary neutral sentences, Croatian usually leaves the subject pronoun out.

Because the verb changes according to the subject.

The infinitive is uzimati = to take.
In the present tense:

• ja uzimam = I take / I am taking
• ti uzimaš = you take
• on/ona/ono uzima = he/she/it takes

So in the sentence:

• uzimam matches the understood subject I
• uzima matches moj brat = my brother

This is standard Croatian verb conjugation.

It can mean either, depending on context.

Croatian present tense often covers both:

• I take
• I am taking

So uzimam košaricu could mean:

• I take a basket
  or
• I’m taking a basket

In this sentence, with danas and the shopping situation, many learners will naturally understand it as a present, real-life action: Today, in the supermarket, I’m taking a basket...

But Croatian does not need a separate continuous form like English does.

Yes. Croatian word order is fairly flexible, because case endings help show the grammatical roles.

The given sentence is natural:

• Danas u supermarketu uzimam košaricu, a moj brat uzima kolica.

But other orders are possible, depending on emphasis, for example:

• U supermarketu danas uzimam košaricu, a moj brat uzima kolica.
• Košaricu danas uzimam u supermarketu, a moj brat uzima kolica.

These alternatives may sound more marked or emphasize different parts, but they are grammatically possible.

The original version is a good neutral choice.

Danas means today and sets the time frame for the whole sentence.

So it tells you that this is happening today, not in general.

Without danas, the sentence would be more neutral and could sound like a general statement or a present-time description depending on context:

• U supermarketu uzimam košaricu, a moj brat uzima kolica.

With danas, the sentence is more specifically tied to today’s situation.

Because moj brat is the subject of the second clause, and subjects are normally in the nominative case.

Here:

• moj brat = my brother
• uzima = takes

So moj brat stays in the nominative.

By contrast, the things being taken are objects, so they use accusative forms:

• košaricu
• kolica (same form as nominative in this case)

Yes, supermarket is used in Croatian as a borrowed word. Croatian uses many international loanwords, especially for modern shops, technology, and everyday urban life.

You may also hear other words depending on context, but supermarket is completely normal and understandable.

In this sentence, it behaves like a Croatian noun and changes by case:

• nominative: supermarket
• locative: supermarketu

So even borrowed words often follow Croatian grammar.