Sljedeći tjedan moja sestra slavi godišnjicu braka, pa joj kupujemo cvijeće.

It is functioning as an accusative time expression, meaning next week.

A useful detail for learners:

• tjedan is a masculine inanimate noun.
• In Croatian, masculine inanimate nouns often have the same form in nominative and accusative singular.
• So sljedeći tjedan looks like nominative, but here it is understood as accusative of time.

Croatian often uses this kind of bare accusative for time expressions:

• ovu subotu = this Saturday
• sljedeći mjesec = next month
• prošli tjedan = last week

You may also hear sljedećeg tjedna, which is also common and means roughly the same thing.

Because Croatian often uses the present tense for a future event when the future time is already clear from context.

Here, Sljedeći tjedan already tells us the action is in the future, so slavi can naturally mean:

• is celebrating next week
• or simply celebrates next week

This is very common with planned, expected, or scheduled events.

So:

• Moja sestra slavi godišnjicu braka sljedeći tjedan. can naturally mean
• My sister is celebrating her wedding anniversary next week.

Yes. slavi is from the verb slaviti, which means to celebrate.

In this sentence:

• moja sestra slavi godišnjicu braka = my sister is celebrating her wedding anniversary

A useful contrast:

• slaviti = to celebrate
• proslaviti = to celebrate fully / mark the occasion / celebrate to completion

In everyday speech, slaviti is perfectly normal here.

Because godišnjicu braka is the direct object of slavi.

The verb slaviti takes an object in the accusative, so:

• nominative: godišnjica
• accusative: godišnjicu

That is why the sentence has:

• slavi godišnjicu braka

Meanwhile, braka is in the genitive, because it means anniversary of marriage.

So the structure is:

• godišnjica = anniversary
• godišnjica braka = anniversary of marriage
• slavi godišnjicu braka = celebrates a wedding anniversary

Because Croatian often links nouns with the genitive where English would use of.

So:

• brak = marriage
• braka = of marriage

That gives:

• godišnjica braka = anniversary of marriage

This is a very common Croatian pattern:

• čaša vode = glass of water
• boja zida = color of the wall
• dan škole = school day / day of the school

Because Croatian often leaves possession understood from context when it is obvious.

In this sentence:

• moja sestra already tells us whose anniversary it is
• so godišnjicu braka is naturally understood as her wedding anniversary

You could make it explicit, for example:

• moja sestra slavi svoju godišnjicu braka

But that usually sounds heavier and is unnecessary unless you want emphasis or contrast.

Not exactly. Here pa means something like:

• so
• and so
• therefore
• which is why

So:

• ..., pa joj kupujemo cvijeće. means
• ..., so we’re buying her flowers.

It links the two ideas with a sense of consequence:

• she has a wedding anniversary
• so we are buying her flowers

It is less neutral than simple i (and), because pa suggests a result or continuation.

Joj is the dative singular pronoun meaning to her or for her.

In this sentence:

• kupujemo joj cvijeće = we are buying her flowers

Croatian uses the dative for the person who receives or benefits from something:

• dajem joj knjigu = I am giving her a book
• kuham mu ručak = I am cooking him lunch
• kupujemo joj cvijeće = we are buying her flowers

So joj corresponds to English her in the sense of for her / to her, not possession.

Because joj is a clitic, and Croatian clitics usually go in the second position in their clause.

In the clause:

• pa joj kupujemo cvijeće

the word pa comes first, so the clitic joj naturally comes right after it.

This is very typical Croatian word order:

• On joj daje knjigu.
• Sutra joj nosimo paket.
• Pa joj kupujemo cvijeće.

Clitics do not usually stand in just any position. Their placement follows special word-order rules.

Because the present tense in Croatian can cover a few meanings depending on context.

Here, kupujemo most naturally suggests:

• we are buying her flowers
• or we’re getting her flowers

It can sound like a present action, a current plan, or an already decided arrangement.

If you said:

• pa ćemo joj kupiti cvijeće

that would sound more explicitly future:

• so we will buy her flowers

Both can work, but kupujemo often feels more natural when the plan is already in motion or seen as straightforward.

Because cvijeće is a very common Croatian noun meaning flowers as a collective or mass noun.

So Croatian often says:

• kupiti cvijeće = to buy flowers

even though English uses a plural count noun.

A few useful comparisons:

• cvijet = a flower
• cvijeće = flowers / floral stuff / a bouquet-type idea

So:

• Kupujemo cvijeće = We’re buying flowers
• Kupujemo tri cvijeta = We’re buying three flowers

Also, cvijeće is a neuter noun, and its nominative and accusative singular are the same, which is why the form does not change here.

Yes. Croatian word order is more flexible than English word order.

This sentence uses a very natural order:

• Sljedeći tjedan moja sestra slavi godišnjicu braka, pa joj kupujemo cvijeće.

But you could move parts around for emphasis, for example:

• Moja sestra slavi godišnjicu braka sljedeći tjedan, pa joj kupujemo cvijeće.
• Cvijeće joj kupujemo jer moja sestra slavi godišnjicu braka sljedeći tjedan.

However, not everything is equally free:

• the clitic joj still needs to follow clitic-placement rules
• some word orders sound more natural than others

So Croatian is flexible, but not random.