U onoj slastičarnici pokraj trga jedan slastičar pravi kolače s lješnjakom i tamnom čokoladom.

Because u means in/at here and expresses a location, so Croatian uses the locative case.

• slastičarnica → slastičarnici
• ona → onoj

Both words change because they have to agree in case, gender, and number.

A useful contrast:

• u onoj slastičarnici = in/at that pastry shop → location
• u onu slastičarnicu = into that pastry shop → movement

Onoj is the feminine singular locative form of onaj, which usually means that.

Here it points to a specific pastry shop, something like in that pastry shop. It sounds more definite than just u slastičarnici.

Compare:

• u slastičarnici = in a/the pastry shop, depending on context
• u onoj slastičarnici = in that pastry shop

They are related words, but one is a person and the other is a place:

• slastičar = pastry chef / confectioner
• slastičarnica = pastry shop / confectionery

This is a useful pattern in Croatian: related words often share a base but have different endings that show whether they mean a person, place, thing, etc.

Because the preposition pokraj requires the genitive case.

So:

• trg → trga

Pokraj means beside, next to, or near.
So pokraj trga means next to / near the square.

Yes, jedan literally means one, but it can also work a bit like English a/an when introducing someone or something.

Since Croatian has no articles, speakers sometimes use jedan to mean:

• one
• a
• a certain

So jedan slastičar can mean one pastry chef or more naturally a pastry chef.

Also, it is often optional. You could also say:

• U onoj slastičarnici pokraj trga slastičar pravi kolače...

That would still be grammatical, but it would sound slightly different in emphasis.

Here pravi is the 3rd person singular present tense of praviti:

• ja pravim = I make
• ti praviš = you make
• on/ona pravi = he/she makes

So jedan slastičar pravi kolače = a pastry chef makes / is making cakes.

A useful extra point: pravi can also be an adjective meaning real / genuine, but not here. In this sentence it is clearly a verb.

It can mean either, depending on context.

Croatian present tense often covers both:

• habitual action: makes
• action happening now: is making

So this sentence could mean:

• he makes cakes there regularly
• he is making cakes there right now

Also, praviti is imperfective, so it presents the action as ongoing, repeated, or not viewed as completed.

Kolače is the accusative plural of kolač.

It is in the accusative because it is the direct object of pravi:

• Što pravi? = What is he making?
• kolače = cakes

Basic forms:

• kolač = singular
• kolači = plural nominative
• kolače = plural accusative

Because the preposition s meaning with takes the instrumental case.

So:

• lješnjak → lješnjakom
• tamna čokolada → tamnom čokoladom

This is very common in Croatian when describing something served or made with an ingredient, accompaniment, or feature.

Croatian often uses the singular when naming an ingredient or flavour in a general way.

So:

• kolači s lješnjakom = cakes with hazelnut / hazelnut-flavoured cakes
• kolači s lješnjacima = cakes with hazelnuts, often suggesting actual whole or visible nuts

So the singular here is natural if the speaker means the ingredient or flavour generally.

Because tamna čokolada has to change to match the instrumental case after s.

• nominative: tamna čokolada
• instrumental: tamnom čokoladom

Both the adjective and the noun change:

• tamna → tamnom
• čokolada → čokoladom

This is called agreement: the adjective must match the noun in gender, number, and case.

Normally, s lješnjakom is the natural form here.

Croatian uses sa instead of s mainly before certain sounds or clusters to make pronunciation easier, especially before words beginning with:

• s
• š
• z
• ž
• or some difficult consonant clusters

For example:

• sa sirom
• sa školom

But with lješnjakom, plain s is the standard choice:

• s lješnjakom

Croatian word order is fairly flexible, much more so than English.

This sentence starts with the location:

• U onoj slastičarnici pokraj trga...

That sets the scene first, which sounds natural.

You could rearrange it, for example:

• Jedan slastičar u onoj slastičarnici pokraj trga pravi kolače...

That is still grammatical, but the emphasis changes a bit. Croatian often moves parts around to highlight:

• place
• person
• action
• contrast

So the original order is natural, but it is not the only possible one.