Pošiljatelj na omotnici bio je moj brat, pa sam odmah otvorila pismo.

Because na here means on in the sense of location, so it requires the locative case.

• omotnica = envelope
• locative singular = omotnici

So:

• na omotnici = on the envelope

If na expressed movement onto something, Croatian would usually use the accusative instead, but here the sender is already written on the envelope, so the locative is used.

Pošiljatelj is in the nominative singular because it is the subject of the sentence.

• pošiljatelj = sender
• nominative singular masculine noun

In this clause:

• Pošiljatelj na omotnici bio je moj brat
• The sender on the envelope was my brother

Both pošiljatelj and moj brat are nominative because they are linked by bio je and identify the same person.

Bio je is the past tense of biti (to be) in the masculine singular.

• bio = masculine singular past active participle of biti
• je = auxiliary is/has, used here to form the past tense

It agrees with pošiljatelj and brat, both masculine singular.

So:

• bio je moj brat = was my brother

If the subject were feminine, you would expect bila je.

Both are possible in Croatian, but word order is flexible.

• bio je moj brat
• je bio moj brat

Both can mean the same thing: was my brother.

In your sentence, bio je sounds natural and smooth after the subject phrase Pošiljatelj na omotnici. Croatian often moves elements around for rhythm, emphasis, or style more freely than English does.

Sam is the 1st person singular auxiliary used to form the past tense.

The full verb phrase is:

• sam otvorila = I opened

Croatian past tense is usually built with:

• auxiliary biti
• past participle

So:

• otvorila sam or sam otvorila = I opened

The subject ja is omitted because Croatian usually does not need subject pronouns when the verb already shows the person.

Because otvorila shows that the speaker is female.

Past participles in Croatian agree with the gender of the subject:

• otvorio = a man opened
• otvorila = a woman opened
• otvorilo = a neuter subject opened

So this sentence tells you that the person speaking is female.

Here pa means something like:

• so
• and so
• therefore
• then

It connects the two parts logically:

• The sender on the envelope was my brother, so I immediately opened the letter.

It is very common in everyday Croatian and often sounds more natural than a more formal connector.

Pismo is the direct object of otvorila, so it is in the accusative case.

But pismo is a neuter noun, and in the singular the nominative and accusative forms are the same:

• nominative: pismo
• accusative: pismo

So even though the form does not change, its function is accusative here.

Odmah means immediately, right away, or at once.

In your sentence:

• pa sam odmah otvorila pismo
• so I immediately opened the letter

Croatian word order is flexible, so odmah can often move:

• Pa sam odmah otvorila pismo.
• Pa sam otvorila pismo odmah.
• Pa odmah otvorih pismo. (more literary or stylistically marked)

The version in your sentence is the most neutral and natural.

In this sentence, yes: omotnica means envelope.

However, learners may also see kuverta, which is another common word for envelope, especially in everyday speech. So:

• omotnica = envelope
• kuverta = envelope

Both are understood, though omotnica may sound a bit more standard or formal.

Moj brat means my brother.

Croatian often uses possessive adjectives like English does when the relationship matters. Here it identifies exactly who the sender was.

• brat = brother
• moj brat = my brother

Without moj, the sentence would mean a/the brother, which would sound less specific and probably unnatural in this context.

Grammatically, pošiljatelj is a masculine noun form. It is commonly used as the standard word for sender.

If you specifically want to refer to a female sender, Croatian can also use:

• pošiljateljica = female sender

But in many contexts, the masculine form can function as the general dictionary form. In this sentence, though, it also matches the fact that the sender turns out to be moj brat, so masculine is clearly appropriate.

The sentence has two clauses joined by pa:

1. Pošiljatelj na omotnici bio je moj brat

   • subject: pošiljatelj na omotnici
   • verb: bio je
   • complement: moj brat

2. pa sam odmah otvorila pismo

   • connector: pa
   • auxiliary: sam
   • adverb: odmah
   • main verb: otvorila
   • object: pismo

A natural literal breakdown is:

• Pošiljatelj = sender
• na omotnici = on the envelope
• bio je = was
• moj brat = my brother
• pa = so
• sam = I have / I did (auxiliary for past tense)
• odmah = immediately
• otvorila = opened
• pismo = letter

So the sentence is a very good example of Croatian case use, past tense formation, and gender agreement.