Pokisnula sam jutros jer nisam ponijela kišobran.

In Croatian past tense, the auxiliary sam is a clitic, and clitics usually go in the second position of the clause.

So in Pokisnula sam jutros..., the participle pokisnula comes first, and sam follows it.

A very common alternative is:

• Jutros sam pokisnula...

That also sounds natural, because sam is still in second position, after jutros.

So the main point is:

• sam usually does not stand freely at the very beginning
• it usually comes after the first stressed element in the clause

The ending -la shows that the speaker is female.

In Croatian past tense, the main verb agrees with the subject in gender and number:

• masculine singular: pokisnuo sam, ponio sam
• feminine singular: pokisnula sam, ponijela sam
• neuter singular: pokisnulo je
• plural forms also change

So this sentence tells you that the speaker is a woman.

A male speaker would say:

• Pokisnuo sam jutros jer nisam ponio kišobran.

The changes are:

• pokisnula → pokisnuo
• ponijela → ponio

The auxiliary sam stays the same, because it marks 1st person singular, not gender.

Pokisnuti means to get caught in the rain or to get rained on, often with the idea of ending up wet because of rain.

So it is not just the general idea to be wet. It specifically connects the wetness to rain.

Compare:

• mokar / mokra = wet
• pokisnuti = to get wet because of rain

That makes pokisnula sam a very natural verb here.

Croatian often drops subject pronouns when they are already clear from the verb form.

So:

• Pokisnula sam already means I got caught in the rain
• you do not need ja

You could say Ja sam pokisnula, but that usually adds emphasis, such as contrast:

• Ja sam pokisnula, ali ona nije.
  I got caught in the rain, but she didn’t.

Without emphasis, leaving out ja is more natural.

Jutros means this morning.

It is an adverb, so it works by itself, without a preposition:

• jutros = this morning
• danas = today
• sinoć = last night / yesterday evening

So:

• Pokisnula sam jutros = I got caught in the rain this morning

You do not need something like u jutro here.

Yes. Both jer and zato što can mean because.

So these are both possible:

• Pokisnula sam jutros jer nisam ponijela kišobran.
• Pokisnula sam jutros zato što nisam ponijela kišobran.

Jer is short and very common.
Zato što is also common and sometimes feels a little fuller or more explicit.

In everyday language, both are normal.

Nisam ponijela means I didn’t bring or I didn’t take along.

It comes from the verb ponijeti, which has the idea of taking something with you.

So:

• ponijeti kišobran = to bring/take an umbrella along
• nisam ponijela kišobran = I didn’t bring an umbrella

This fits the sentence well: she got caught in the rain because she failed to take an umbrella with her.

In Croatian, negation with biti in forms like sam, si, je is usually built as a single word:

• nisam = I am not / I did not
• nisi = you are not / you did not
• nije = he/she/it is not / did not

So the negative past tense is formed with:

• nisam + participle
• nisi + participle
• nije + participle, etc.

That is why you say:

• nisam ponijela
• not ne sam ponijela

Because kišobran is a masculine inanimate noun, and in the singular its accusative form is the same as the nominative.

The verb ponijeti takes a direct object, so kišobran is in the accusative:

• nominative: kišobran
• accusative: kišobran

That is why it looks unchanged.

Compare with a feminine noun:

• nisam ponijela torbu
  Here torba changes to torbu in the accusative.

Croatian has no articles, so kišobran can mean either an umbrella or the umbrella, depending on context.

So this sentence could mean:

• I didn’t bring an umbrella
• I didn’t bring the umbrella

Usually, context tells you which one is meant. If the meaning has already been given to you as an umbrella, that is a perfectly natural translation.

Yes, both are perfective here.

• pokisnuti = to get caught in the rain, to become rain-soaked
• ponijeti = to take/bring along

Perfective verbs usually present an action as a completed whole.

That fits this sentence well:

• getting caught in the rain happened as a complete event
• not bringing the umbrella refers to the completed fact that she left without it

Their imperfective partners would be different verbs, depending on meaning and context.

Yes. Croatian word order is fairly flexible, though not completely free.

These are all possible, with slightly different emphasis:

• Pokisnula sam jutros jer nisam ponijela kišobran.
• Jutros sam pokisnula jer nisam ponijela kišobran.
• Jer nisam ponijela kišobran, pokisnula sam jutros.

The most neutral everyday versions are probably:

• Pokisnula sam jutros...
• Jutros sam pokisnula...

The basic meaning stays the same, but the first position often gives extra focus to that element.

Yes. It means that not bringing the umbrella is presented as the reason she got rained on.

Structurally:

• Pokisnula sam jutros = main clause
• jer nisam ponijela kišobran = reason clause

So the sentence is a very straightforward result + cause pattern:

• result: she got wet in the rain
• cause: she didn’t bring an umbrella

Yes, it sounds completely natural.

It is a very normal colloquial and standard way to say:

• I got caught in the rain this morning because I didn’t bring an umbrella.

Nothing in the sentence sounds unusual or overly formal.