Kad stignemo na odredište, javit ću ti se da znaš da je sve prošlo dobro.

Because Kad stignemo na odredište is a subordinate time clause: When we arrive at the destination. In Croatian, when this kind of clause comes before the main clause, it is normally separated by a comma.

So the structure is:

• Kad stignemo na odredište, = subordinate clause
• javit ću ti se... = main clause

This is very common in Croatian. After time words like kad (when), Croatian often uses the present tense to refer to a future event.

So:

• Kad stignemo... = When we arrive...
• not When we are arriving...

Also, stići is a perfective verb, so its present form often points to a completed future action in this kind of sentence.

Compare:

• Kad dođem, nazvat ću te. = When I arrive, I’ll call you.

They both mean when.

• kad is shorter and very common in everyday speech
• kada is a bit fuller, sometimes slightly more formal or more emphatic

In this sentence, kad is completely natural.

Because Croatian does not have articles like a and the.

So odredište can mean:

• a destination
• the destination
• sometimes even our destination

The exact meaning comes from context. In this sentence, it is naturally understood as the destination.

Because na odredište shows movement toward a goal, so odredište is in the accusative case.

• na odredište = to the destination
• na odredištu = at the destination

So:

• stignemo na odredište = we arrive at/reach the destination
• smo na odredištu = we are at the destination

This is mostly a matter of normal Croatian usage with this noun and verb combination. Croatian commonly says:

• stići na odredište

Even though English says arrive at the destination, Croatian uses na here as the standard collocation. It is best learned as a set phrase.

Javiti se is a reflexive verb expression that means something like:

• to contact
• to get in touch
• to check in
• to let someone know

So javit ću ti se means I’ll contact you / I’ll get in touch with you / I’ll let you know.

The se is part of the verb expression javiti se. It is not translated word-for-word in English.

Also:

• ti = to you

So literally the whole chunk is roughly I will contact myself to you, but that literal breakdown is not how you should understand it. Treat javiti se as one unit.

In standard Croatian future tense, when the auxiliary ću comes after the infinitive, the final -i of the infinitive is dropped in writing:

• javiti + ću → javit ću
• raditi + ću → radit ću
• pisati + ću → pisat ću

So javit ću is the standard written form.

You can also place the auxiliary before the infinitive in some contexts, and then the full infinitive stays:

• Sutra ću ti se javiti.

But javiti ću is not the standard Croatian spelling.

Because ću, ti, and se are all clitic forms: short, unstressed words that follow special word-order rules in Croatian.

In this sentence, they appear in the normal clitic cluster:

• ću = future auxiliary
• ti = dative pronoun, to you
• se = reflexive particle

So:

• javit ću ti se = correct

This order is very typical and sounds natural to native speakers.

Because the two das do different jobs.

The first da means something like so that:

• javit ću ti se da znaš...
• I’ll contact you so that you know...

The second da means that and introduces the content of know:

• da znaš da je sve prošlo dobro
• so that you know that everything went well

So the sentence has:

• da znaš = so that you know
• da je sve prošlo dobro = that everything went well

This is the perfect tense:

• je = auxiliary is/has
• prošlo = past participle

Together:

• je prošlo = has gone / went

So da je sve prošlo dobro means that everything went well or that everything has gone well.

Why use a past form inside a future sentence? Because from the moment of javit ću ti se (I’ll contact you), the event will already be completed, so Croatian naturally uses the perfect.

As for word order, je is a clitic, and clitics in Croatian usually come very early in the clause. After da, it is natural to get:

• da je sve prošlo dobro

Because the subject is sve (everything), and sve normally takes neuter singular agreement in this kind of sentence.

So:

• sve je prošlo dobro = everything went well

That is why the participle is:

• prošlo = neuter singular

Compare:

• On je prošao dobro. = He came through well / It went well for him.
• Ona je prošla dobro. = She came through well / It went well for her.
• Sve je prošlo dobro. = Everything went well.

Here it functions as an adverb, meaning well.

So:

• sve je prošlo dobro = everything went well

Even though dobro has the same form as the neuter adjective good, in this sentence it modifies the verb idea prošlo (went / passed), so it is understood as well, not good.