Kad stignemo na obalu, prvo ću otvoriti suncobran i obući kupaći kostim.

In Croatian, after time words such as kad (when), you normally do not use the future auxiliary ću/ćeš/će... the way English sometimes would.

So Croatian says:

• Kad stignemo na obalu... = When we arrive at the shore...

Literally, stignemo looks like a present-tense form, but here it refers to a future event because it comes after kad.

This is especially common with perfective verbs, and stići is perfective. So:

• kad stignemo = when we arrive / once we arrive

Using kad ćemo stići would usually not fit this sentence.

Stignemo is the 1st person plural present tense form of stići:

• ja stignem = I arrive
• mi stignemo = we arrive

In this sentence, it means we arrive in the sense of a future completed event, because it is used after kad.

So:

• Kad stignemo na obalu... = When we arrive at the shore...

Even though it is formally present tense, the meaning here is future.

Because Croatian uses different cases depending on whether there is:

• movement toward a place
• or location at a place

Here, stignemo na obalu means we arrive to the shore, so there is motion toward a destination. That is why na takes the accusative:

• na obalu = to the shore

Compare:

• na obali = on/at the shore → location, so locative
• na obalu = onto/to the shore → destination, so accusative

This is the normal Croatian future I construction:

• ću = future auxiliary
• otvoriti = infinitive

So:

• otvoriti = to open
• ću otvoriti = I will open

The full part:

• prvo ću otvoriti suncobran = first I will open the sun umbrella

Croatian future I is usually formed like this:

• ja ću otvoriti
• ti ćeš otvoriti
• on/ona/ono će otvoriti
• mi ćemo otvoriti
• vi ćete otvoriti
• oni/one/ona će otvoriti

Because the sentence has two different clause types:

1. Kad stignemo na obalu
   This is a time clause introduced by kad, and Croatian normally uses a present form here.

2. prvo ću otvoriti suncobran i obući kupaći kostim
   This is the main clause, so Croatian uses the normal future form with ću.

So the pattern is:

• When we arrive..., I will...

In Croatian:

• Kad + present/perfective form
• main clause + future

Yes, both are possible.

Croatian future auxiliaries such as ću are clitics, and clitics usually go near the beginning of the clause, often in the second position.

So this sentence has:

• prvo ću otvoriti...

That is very natural.

You can also sometimes place the infinitive first and attach the auxiliary after it:

• otvorit ću

So both of these are possible in many contexts:

• Prvo ću otvoriti suncobran.
• Prvo otvorit ću suncobran. → less natural
• Prvo ću otvorit suncobran. → colloquial/informal in some usage
• Otvorit ću suncobran. → very common

In your sentence, prvo ću otvoriti... is the most standard and natural choice.

Because both infinitives depend on the same future auxiliary.

So:

• ću otvoriti
• i obući

means:

• I will open
• and put on

Croatian does not need to repeat ću if the same subject and the same future auxiliary apply to both verbs.

So this is perfectly normal:

• Prvo ću otvoriti suncobran i obući kupaći kostim.

You could repeat it for emphasis or clarity, but it is usually unnecessary:

• Prvo ću otvoriti suncobran i ću obući kupaći kostim. → not natural

So the single ću covers both actions.

Obući means to put on / to get dressed in.

In this sentence:

• obući kupaći kostim = to put on a swimsuit

This verb focuses on the action of putting clothes on.

Useful comparison:

• obući = to put on, to dress oneself in
• nositi = to wear, to have on
• skinuti = to take off

So:

• Obući ću kupaći kostim. = I will put on a swimsuit.
• Nosim kupaći kostim. = I am wearing a swimsuit.

Both are direct objects, so they are in the accusative case:

• otvoriti suncobran
• obući kupaći kostim

However, both nouns are masculine inanimate, and for masculine inanimate nouns, the accusative singular is usually the same as the nominative singular.

So:

• suncobran
  nominative: suncobran
  accusative: suncobran

• kostim
  nominative: kostim
  accusative: kostim

That is why they look unchanged.

The adjective kupaći also agrees with kostim, and in masculine inanimate accusative singular it also looks the same as nominative:

• kupaći kostim

Prvo means first or firstly.

Here it tells you the order of actions:

• Kad stignemo na obalu, prvo ću otvoriti suncobran...
• When we arrive at the shore, first I’ll open the umbrella...

Its position is somewhat flexible, but not every placement sounds equally natural. Common options include:

• Kad stignemo na obalu, prvo ću otvoriti suncobran...
• Prvo ću otvoriti suncobran kad stignemo na obalu...
• Kad stignemo na obalu, otvorit ću prvo suncobran... → possible, but less natural here

The version in your sentence is very natural.

They both mean when.

• kad = shorter, very common in everyday speech
• kada = slightly fuller/formal or simply stylistic

In most situations, they are interchangeable:

• Kad stignemo na obalu...
• Kada stignemo na obalu...

Both are correct.

Learners will often hear kad more often in conversation.

Yes, Croatian word order is more flexible than English word order, although some versions sound more natural than others.

The given sentence:

• Kad stignemo na obalu, prvo ću otvoriti suncobran i obući kupaći kostim.

Possible variations:

• Prvo ću otvoriti suncobran i obući kupaći kostim kad stignemo na obalu.
• Kad stignemo na obalu, suncobran ću prvo otvoriti i obući kupaći kostim. → grammatically possible, but less natural
• Kad stignemo na obalu, otvorit ću prvo suncobran i obući kupaći kostim.

Croatian allows movement of elements for emphasis, rhythm, or style, but the original sentence is a very natural neutral version.

Yes. Otvoriti suncobran literally means to open a sun umbrella / parasol.

• otvoriti = to open
• suncobran = sunshade, sun umbrella, parasol

So the phrase is completely normal in Croatian, just like open an umbrella in English.

You may also notice that suncobran is made from:

• sunce = sun
• braniti = to defend/protect

So the idea is something that protects you from the sun.

Yes, kupaći kostim is a normal and standard way to say swimsuit.

You may also hear shorter everyday forms such as:

• kupaći = swimsuit / bathing suit

For example:

• Obuci kupaći. = Put on your swimsuit.

But in a full neutral sentence, kupaći kostim is very clear and standard.

So in your sentence:

• obući kupaći kostim = to put on a swimsuit