Kad stignemo na obalu, želim se odmah osvježiti u moru, a poslije sjediti u hladovini.

Both kad and kada mean when.

• kad is shorter and very common in everyday speech and writing
• kada is a bit more formal or emphatic

So Kad stignemo na obalu... is completely natural and normal.

In Croatian, after time words like kad (when), it is very common to use the present tense to talk about a future event.

So:

• Kad stignemo na obalu... = When we get/reach the coast...

English often uses the present in this kind of clause too: When we arrive..., not When we will arrive...

Also, stignemo is from the perfective verb stići, which often refers to a completed future action in this kind of structure.

Stignemo is the 1st person plural present form of stići.

Here it means something like:

• we arrive
• we get there
• we reach

In this sentence, stignemo na obalu is best understood as when we reach/arrive at the coast/shore.

Because na obalu shows movement toward a place, so Croatian uses the accusative.

• na obalu = to the shore/coast
• na obali = on the shore/coast

So:

• stignemo na obalu = we arrive at / come to the shore
• sjedimo na obali = we are sitting on the shore

This is a very common Croatian pattern:

• u grad = into/to the city
• u gradu = in the city
• na stol = onto the table
• na stolu = on the table

Because Kad stignemo na obalu is a subordinate time clause at the beginning of the sentence.

Croatian normally separates that kind of introductory clause with a comma:

• Kad stignemo na obalu, želim...

There is also a comma before a because a connects the two infinitive ideas:

• osvježiti se u moru
• poslije sjediti u hladovini

The verb osvježiti se is a reflexive verb, meaning to refresh oneself, or more naturally in English here, to cool off / to freshen up.

So:

• osvježiti = to refresh something
• osvježiti se = to refresh oneself / to cool off

In this sentence, the person wants to go into the sea and cool off, so se is necessary.

After želim (I want), Croatian normally uses the infinitive.

So:

• želim se osvježiti = I want to cool off
• želim sjediti = I want to sit

This is very similar to English want + to + verb, except Croatian uses the infinitive directly, without a separate word like to.

Because u moru expresses location: being in the sea.

• u moru = in the sea

Croatian uses the locative after u when it means location.

If it meant movement into the sea, you could get the accusative:

• skočiti u more = to jump into the sea

But here the idea is refreshing oneself while in the sea, so u moru is the natural choice.

A here links two ideas and can mean something like:

• and then
• and afterwards
• while after that
• sometimes simply and

Poslije means afterwards / later.

So:

• a poslije sjediti u hladovini = and afterwards sit in the shade

The a gives a nice transition from the first activity to the next one. It is slightly different from plain i because it helps separate the two stages more clearly.

This is a very important aspect difference.

• sjesti = to sit down (a single completed action; perfective)
• sjediti = to be sitting / to sit (an ongoing state or activity; imperfective)

Here the speaker wants:

1. to cool off in the sea
2. then to spend time sitting in the shade

That is why sjediti is used. The focus is on the activity/state of sitting, not just the moment of sitting down.

If the sentence were about the act of taking a seat, sjesti would make more sense.

Because one želim can govern both infinitives:

• želim se odmah osvježiti
• (želim) poslije sjediti u hladovini

Croatian often omits the repeated verb when it is clearly understood.

So the full meaning is:

• I want to cool off immediately in the sea, and afterwards (I want to) sit in the shade.

Because se is a clitic, and Croatian clitics usually come very early in the clause, typically after the first stressed element.

So:

• želim se odmah osvježiti = natural
• želim odmah se osvježiti = much less natural
• želim osvježiti se = generally unnatural in standard usage

English speakers often expect se to behave like self, but it does not. Its position follows Croatian clitic word order, not English logic.

Odmah means:

• immediately
• right away

So želim se odmah osvježiti u moru means the speaker wants to do that as soon as they reach the shore, without delay.

It gives the sentence a more vivid, natural feeling: first thing on arrival, the person wants to get into the sea and cool off.

Both relate to shade or coolness, but hladovina very often means shade in the sense of a shady place.

So:

• u hladovini = in the shade

This is a very natural phrase in Croatian.

You may also hear u hladu, which also means in the shade. In many contexts, both are possible, but u hladovini often sounds especially like sitting in a shaded area, for example under trees or an awning.