U ovoj trgovini prodavač pomaže kupcima, a blagajnik radi do osam.

Because u can take different cases depending on meaning.

• u ovoj trgovini = in this shop/store → location, so Croatian uses the locative
• u ovu trgovinu = into this shop/store → movement toward a destination, so Croatian uses the accusative

In this sentence, the meaning is location, so u ovoj trgovini is correct.

It is locative singular.

• trgovina = shop/store
• locative singular: trgovini
• ovoj is the matching form of ovaj (this) for a feminine singular noun in the locative

So u ovoj trgovini literally means in this store.

Because the verb pomagati (to help) takes the dative case in Croatian.

So:

• pomaže kupcima = helps customers
• not pomaže kupce

This is different from English, where help takes a direct object. In Croatian, you help to/for customers, grammatically speaking, so the dative is used.

Because kupac has a plural stem change.

Its forms are:

• singular: kupac
• nominative plural: kupci
• dative plural: kupcima

Many masculine nouns ending in -ac behave like this. The -a- disappears in some forms, so kupac becomes kupc- before endings like -i or -ima.

They are related jobs, but not the same one.

• prodavač = salesperson, shop assistant, sales clerk
• blagajnik = cashier

So the sentence distinguishes between someone who helps customers and someone who works at the register.

If the worker is female, you will often hear:

• prodavačica
• blagajnica

A often means and, but with a sense of contrast, comparison, or topic shift.

Here it connects two related facts:

• the salesperson helps customers
• the cashier works until eight

So a sounds a bit like:

• and
• while
• whereas

If you used i, it would sound more like simple addition, with less contrast between the two roles.

Yes, radi do osam means works until eight, and sati (o’clock / hours) is understood.

So:

• do osam = until eight
• full version: do osam sati

In everyday Croatian, leaving out sati is very common when the meaning is obvious.

They are both present tense, 3rd person singular.

• pomaže = he/she helps or is helping
• radi = he/she works or is working

In this sentence, the present tense most naturally gives a general/habitual meaning, like describing how things work in that store.

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

So:

• prodavač can mean a salesperson or the salesperson
• blagajnik can mean a cashier or the cashier

The exact meaning depends on context. Croatian uses word order, context, and words like ovaj (this) when it needs to be more specific.

No, Croatian word order is fairly flexible because cases show the grammatical relationships.

The sentence begins with U ovoj trgovini to set the scene: In this store...

You could rearrange parts of the sentence, but the emphasis would change. For example:

• Prodavač u ovoj trgovini pomaže kupcima...

This is still understandable, but the original version sounds very natural because it introduces the location first.

Croatian often uses the singular to talk about a typical person in a role, not necessarily one specific individual.

So here the singular can mean:

• the salesperson, as a role in that store
• the cashier, as a role in that store

If you wanted plural, you would say something like:

• U ovoj trgovini prodavači pomažu kupcima, a blagajnici rade do osam.

That would mean there are multiple salespeople and multiple cashiers.