Ako pošiljka ne stigne sutra, zvat ću ih odmah.

Because after ako (if), Croatian normally uses the present tense to talk about a real future condition.

So:

• Ako pošiljka ne stigne sutra... = If the shipment doesn’t arrive tomorrow...
• not usually Ako pošiljka neće stići sutra...

This is actually similar to English, which also says:

• If it doesn’t arrive tomorrow, I’ll call them
• not normally If it won’t arrive tomorrow, I’ll call them

So the form is present, but the meaning is future because of the whole sentence pattern.

Stigne and stiže come from different aspectual verbs.

• stigne = from stići (to arrive, perfective)
• stiže = from stizati (to be arriving / to arrive repeatedly, imperfective)

In this sentence, the speaker is talking about one completed event: whether the shipment arrives tomorrow or not. That is why Croatian uses the perfective verb stići, in the form stigne.

So:

• ne stigne sutra = doesn’t arrive tomorrow / doesn’t get there tomorrow
• ne stiže sutra would sound more like isn’t arriving tomorrow or could suggest a more ongoing/planned sense

For a single expected delivery, stigne is the normal choice.

You can say it in some contexts, but it usually does not sound like the most neutral version of this sentence.

The standard, natural pattern for a future condition is:

• Ako + present tense, then main clause in the future

So:

• Ako pošiljka ne stigne sutra, zvat ću ih odmah.

Using neće stići inside the ako clause often sounds more marked, as if you mean:

• if it turns out that it is not going to arrive tomorrow
• if it won’t be arriving tomorrow

That is a slightly different feel. For the ordinary if X happens tomorrow pattern, ne stigne is better.

Because in standard Croatian, when the infinitive comes before the future auxiliary (ću, ćeš, će...), infinitives ending in -ti usually drop the final -i.

So:

• zvati
  • ću → zvat ću

This is standard.

You can also put the auxiliary first:

• Ja ću zvati
• Zvat ću

Both are correct, but:

• zvat ću = correct
• zvati ću = non-standard

Yes. That is also correct.

Croatian allows more than one word order here:

• Zvat ću ih odmah.
• Ja ću ih zvati odmah.
• Odmah ću ih zvati.

The basic meaning stays the same, but the emphasis can shift a little.

• Zvat ću ih odmah sounds neutral and natural.
• Ja ću ih zvati odmah puts a bit more focus on I.
• Odmah ću ih zvati puts more focus on immediately.

Also, ću and ih are clitic forms, so they tend to appear near the beginning of their clause, not in just any position.

Because ih is the normal unstressed clitic form for them here.

In this sentence, ih is the direct object of zvati, so it means them.

• zvat ću ih = I will call them

njih is the full, stressed form. It is used when:

• you want emphasis or contrast
• it comes after a preposition

For example:

• Zvat ću njih, a ne vas. = I’ll call them, not you.
• bez njih = without them

So in a neutral sentence like this one, ih is the normal choice.

Here it means to call.

• zvati nekoga = to call someone
• zvati se = to be called / to be named

So:

• Zvat ću ih odmah. = I’ll call them immediately.
• Zovem se Ana. = My name is Ana / I am called Ana.

The pronoun ih makes it clear that this is the non-reflexive meaning: call them.

Pošiljka usually means something like:

• shipment
• parcel
• package
• consignment

The best English choice depends on context.

Grammatically, here it is:

• nominative singular
• feminine
• the subject of stigne

So pošiljka ne stigne literally means the shipment does not arrive.

Because the sentence begins with a subordinate ako clause.

Structure:

• Ako pošiljka ne stigne sutra, = subordinate clause
• zvat ću ih odmah. = main clause

In Croatian, when a dependent clause comes first, it is normally separated from the main clause with a comma.

So the comma here is standard and expected.

Because Croatian has no articles.

There is no direct equivalent of English a or the in ordinary Croatian grammar. Whether something means a shipment or the shipment is understood from context.

So:

• pošiljka can mean a shipment or the shipment

In this sentence, the context probably makes it definite in English:

• If the shipment doesn’t arrive tomorrow...

Because sutra is already a time adverb meaning tomorrow.

Croatian often uses time words directly:

• danas = today
• sutra = tomorrow
• jučer = yesterday

So:

• stigne sutra = arrives tomorrow

No preposition is needed.

If you changed the meaning, you might use a preposition:

• do sutra = by tomorrow
• od sutra = from tomorrow

But plain sutra simply means tomorrow.

Because Croatian often leaves subject pronouns out when they are already clear from the verb form.

• zvat ću already shows I will call
• so ja is not necessary

You can add ja if you want emphasis:

• Ja ću ih zvati odmah.

But the neutral version usually omits it:

• Zvat ću ih odmah.

This is very common in Croatian.