Večeras moram spakirati torbu za sutrašnji put u Zagreb.

Večeras means this evening / tonight. It is an adverb of time, so it tells you when the action happens.

In this sentence, it modifies moram spakirati:
Večeras moram spakirati... = Tonight I have to pack...

It is different from večer (evening), which is a noun.

Croatian usually does not need an explicit subject pronoun when the verb already shows who is doing the action.

Here, moram means I must / I have to, so ja (I) is understood automatically.

You could say Ja večeras moram spakirati torbu..., but ja is usually added only for emphasis, contrast, or clarity.

Moram is the 1st person singular present tense of morati, meaning must / have to.

So:

• moram = I must / I have to
• moraš = you must
• mora = he/she/it must

After morati, Croatian normally uses an infinitive, which is why you get moram spakirati.

Because after modal verbs like morati (must / have to), Croatian uses the infinitive.

So:

• moram spakirati = I have to pack
• moram ići = I have to go
• moram učiti = I have to study

The infinitive here is spakirati.

This is a very important Croatian verb pair:

• pakirati = to pack, imperfective
• spakirati = to pack up / finish packing, perfective

In this sentence, spakirati is used because the speaker means completing the action: the bag needs to be packed, not just packed in general.

So:

• Moram pakirati torbu. = I have to be packing / I have to pack (process-focused)
• Moram spakirati torbu. = I have to pack the bag completely (result-focused)

In everyday speech, spakirati is very natural here.

Because torba is the direct object of the verb spakirati, so it goes into the accusative case.

• nominative: torba = bag
• accusative: torbu = bag (as the object)

So:

• Torba je velika. = The bag is big.
• Spakirati torbu. = To pack the bag.

Since torba is a feminine noun, the accusative singular ends in -u.

Za here means for, and after za in this meaning, Croatian uses the accusative.

So:

• za put = for the trip
• za sutrašnji put = for tomorrow’s trip

The adjective sutrašnji (tomorrow’s) agrees with put in gender, number, and case.

Because put is a masculine inanimate noun, its accusative singular looks the same as the nominative:

• nominative: sutrašnji put
• accusative: sutrašnji put

So the form does not visibly change, but it is still accusative because of za.

• sutra = tomorrow
• sutrašnji = tomorrow’s / relating to tomorrow

So:

• put sutra would not be the normal way to say tomorrow’s trip
• sutrašnji put is the natural adjectival phrase: tomorrow’s trip

In this sentence, Croatian uses the adjective because it modifies the noun put.

Because Croatian distinguishes between:

• motion toward a place → u + accusative
• location in a place → u + locative

Here the trip is to Zagreb, so there is motion:

• put u Zagreb = trip to Zagreb

Compare:

• Idem u Zagreb. = I’m going to Zagreb.
• Živim u Zagrebu. = I live in Zagreb.

So u Zagreb is correct because the destination is being expressed.

Because with motion, u takes the accusative, and for some masculine inanimate nouns and many place names, the accusative looks the same as the nominative.

So:

• nominative: Zagreb
• accusative: Zagreb
• locative: Zagrebu

That is why you see:

• u Zagreb = to Zagreb
• u Zagrebu = in Zagreb

Yes. Croatian word order is more flexible than English word order.

The neutral order here is very natural:

Večeras moram spakirati torbu za sutrašnji put u Zagreb.

But other orders are possible depending on emphasis, for example:

• Moram večeras spakirati torbu za sutrašnji put u Zagreb.
• Torbu za sutrašnji put u Zagreb moram spakirati večeras.

The meaning stays basically the same, but the focus shifts. Croatian often uses word order to highlight what is new, important, or contrasted.

Not really in the English sense of into. Croatian u covers both in and into, depending on whether the meaning is location or movement.

So:

• u Zagrebu = in Zagreb
• u Zagreb = to/into Zagreb

In English, trip to Zagreb sounds best. Croatian uses u because the trip is directed toward the city.

Yes, that is also possible in many contexts.

• spakirati torbu = to pack the bag
• spremiti torbu = to get the bag ready / put it in order / pack it

In everyday speech, both can work, but spakirati torbu is especially clear when you mean packing for a trip.

Yes, it is natural and idiomatic.

It sounds like a normal everyday sentence someone would say when preparing for travel:

Večeras moram spakirati torbu za sutrašnji put u Zagreb.

A speaker might also choose slightly different wording, but this version is completely normal and correct.