Najprije ću završiti poruku, a poslije ću te zvati.

Croatian often leaves out subject pronouns when they are obvious from the verb form.

In ću završiti and ću te zvati, the form ću already tells you the subject is I. So Ja ću... is possible, but usually unnecessary unless you want emphasis or contrast.

• Ja ću završiti poruku = I will finish the message
• Najprije ću završiti poruku = First, I’ll finish the message

Both are correct, but the version without ja is more neutral and natural.

Najprije means first, at first, or first of all in a sequence of actions.

In this sentence, it introduces the first thing the speaker will do:

• Najprije ću završiti poruku = First I’ll finish the message

A very common near-synonym is prvo. In many everyday contexts, najprije and prvo can both work.

• Najprije ću završiti poruku
• Prvo ću završiti poruku

Both sound natural.

Here poslije means afterwards or later/after that.

So:

• a poslije ću te zvati = and afterwards I’ll call you

A similar word is kasnije, which usually means later. In many cases both are possible, but there is a slight difference in feel:

• poslije = after that, afterwards
• kasnije = later, at a later time

So this sentence could also be:

• Najprije ću završiti poruku, a kasnije ću te zvati.

That is also natural.

Croatian future tense 1 is commonly formed with:

• the clitic form of htjeti (ću, ćeš, će, ćemo, ćete, će)
• plus the infinitive

So:

• ću završiti = I will finish
• ću zvati = I will call

The dictionary forms are:

• završiti = to finish
• zvati = to call

So the sentence uses two future forms:

• ću završiti
• ću zvati

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

So in:

• Najprije ću završiti poruku
• a poslije ću te zvati

the first element is najprije or poslije, and then the clitic ću comes right after it.

This is very normal Croatian word order.

Compare:

• Ja ću završiti poruku.
• Najprije ću završiti poruku.
• Sutra ću završiti poruku.

In each case, ću tends to come after the first stressed element.

Yes. That is also correct.

Croatian has two common ways to express this future:

• ću završiti
• završit ću

And similarly:

• ću zvati
• zvat ću

Both patterns are standard. In your sentence, because najprije and poslije come first, ću završiti and ću zvati fit naturally.

So these are both possible:

• Najprije ću završiti poruku
• Najprije završit ću poruku — this is not the normal word order

If the infinitive comes first, then you usually get:

• Završit ću poruku.

So the main point is: both future patterns exist, but clitic placement affects what sounds natural.

Because poruku is the accusative singular form of poruka.

• poruka = message (basic/dictionary form, nominative)
• poruku = message (direct object, accusative)

In the sentence, poruku is the thing being finished, so it is the direct object:

• završiti poruku = to finish the message

This is a very common pattern with feminine nouns ending in -a:

• nominative: poruka
• accusative: poruku

Because te is the accusative form of you, and zvati takes a direct object.

• zvati te = to call you

By contrast, ti usually means to you and is dative, not accusative.

Compare:

• Zvat ću te. = I’ll call you.
• Dat ću ti poruku. = I’ll give you a message.

So in this sentence, te is correct because you are the person being called.

Yes, zvati can mean different things depending on context:

1. to call someone
2. to be called / to have the name

Examples:

• Zvat ću te poslije. = I’ll call you later.
• Kako se zoveš? = What’s your name? / What are you called?

In your sentence, because of the context and meaning, zvati te clearly means call you, usually by phone.

A often links two clauses while showing a mild contrast or a shift from one action to another. Here it works a bit like:

• and then
• while/as for the next thing
• sometimes just a natural clause link

So:

• Najprije ću završiti poruku, a poslije ću te zvati.

means something like:

• First I’ll finish the message, and afterwards I’ll call you.

You might also see pa or just two clauses side by side in other contexts, but a sounds very natural here because it neatly separates the two steps.

Because both actions are in the future:

1. finishing the message
2. calling the person

Croatian normally marks both of them as future here:

• ću završiti
• ću te zvati

English sometimes uses different patterns in time clauses, but in a simple coordination like this, Croatian straightforwardly uses future for both actions.

Both can be possible, but they are not identical in feel.

• zvati = to call, to be calling
• nazvati = to call, often with a clearer sense of completing the call or making the call once

So you might hear:

• Poslije ću te zvati.
• Poslije ću te nazvati.

Both are natural. In many everyday situations, nazvati sounds especially common for I’ll call you. But zvati is still perfectly understandable and correct.

Croatian word order is fairly flexible, but not completely free. This sentence is very natural as it stands.

Possible variations include:

• Najprije ću završiti poruku, a poslije ću te zvati.
• Poruku ću najprije završiti, a poslije ću te zvati.
• Najprije ću poruku završiti, a poslije ću te zvati.

These may shift emphasis slightly, but the original version is probably the most neutral and idiomatic.

The main thing to keep in mind is that clitics like ću and te have preferred positions, so not every imaginable rearrangement will sound natural.