Pošiljka je stigla jutros, ali mi je nisu ostavili u sandučiću nego kod susjede.

Because pošiljka is a feminine singular noun. In Croatian, the past participle agrees with the subject in gender and number.

So:

• pošiljka = feminine singular
• je stigla = has arrived / arrived

If the subject were a masculine noun like paket, you would say Paket je stigao.

Jutros is an adverb meaning this morning, specifically earlier today in the morning.

It is different from:

• ujutro = in the morning in a more general sense
• svako jutro = every morning

So stigla jutros means the arrival happened this morning.

Croatian often drops subject pronouns when the verb already makes the person and number clear.

• nisu ostavili = they did not leave

English usually needs they, but Croatian does not. The speaker does not have to say oni unless they want special emphasis.

Mi is the dative singular of ja and means to me / for me.

Here it marks the person affected by the action. Croatian uses this kind of dative much more naturally than English does.

So ali mi je nisu ostavili... is roughly:

• but they didn’t leave it for me / to me...

If you removed mi, the sentence would still work, but it would sound less personal.

In this clause, je does not mean is. It is the short object pronoun it, referring back to pošiljka.

So the pieces are:

• mi = to me / for me
• je = it
• nisu ostavili = they did not leave

So the structure is essentially they didn’t leave it for me...

This is a very common Croatian feature: the same form je can be either:

• an auxiliary verb, as in je stigla
• or an object pronoun, as in mi je nisu ostavili

Ostavili is the past participle plural masculine form of ostaviti.

It agrees with the understood subject, which is an omitted they — probably delivery workers, couriers, or some other people responsible for the package.

Croatian normally uses masculine plural for:

• a mixed group
• an unknown group
• a group referred to generally

That is why you get nisu ostavili, not a feminine singular form.

Because u takes the locative when it means in as a location.

Here the meaning is static location:

• u sandučiću = in the mailbox

The base noun is sandučić, and its locative singular is sandučiću.

Compare:

• u sandučiću = in the mailbox (location)
• u sandučić = into the mailbox (movement)

Kod + genitive is a very common Croatian way to say:

• at someone’s place
• with someone
• in someone’s care

So kod susjede means at the neighbor’s place / with the neighbor.

Here susjede is genitive singular of susjeda, which here is female neighbor.

Useful comparison:

• kod susjede = at the female neighbor’s place
• kod susjeda = at the male neighbor’s place

Because Croatian commonly uses ne ... nego ... for not ... but rather ...

In this sentence:

• nisu ostavili u sandučiću nego kod susjede
• they didn’t leave it in the mailbox, but rather at the neighbor’s

So the sentence has two different contrast words doing two different jobs:

• ali = connects the two larger clauses
• nego = gives the corrected alternative after a negation

Yes. Croatian has fairly flexible word order, but short unstressed words such as mi and je tend to appear near the beginning of the clause.

That is why the middle of the sentence looks compact:

• ali mi je nisu ostavili

A learner may expect something more like English word order, but Croatian often groups these short forms together early in the sentence.

So this order is natural Croatian, even though it can look unusual at first.