Ja sam juče poslala pismo prijateljici.

Yes, Serbian often drops subject pronouns because the verb already shows the person.

So Ja sam juče poslala... can often be shortened to:

Juče sam poslala pismo prijateljici.

Including ja usually adds a little emphasis or contrast, like:

• I sent the letter yesterday.
• I was the one who sent it.

So ja is not wrong at all—it just is not always necessary.

Sam is the 1st person singular present form of biti (to be), and here it is used as an auxiliary verb to form the past tense.

So the past tense here is built like this:

• sam = auxiliary
• poslala = past participle

Together:

• sam poslala = I sent

This is the normal way Serbian forms the past tense.

Because poslala shows that the speaker is female.

In Serbian past tense, the participle agrees with the subject in gender and number.

For ja:

• male speaker: poslao
• female speaker: poslala

So:

• Ja sam juče poslao pismo prijateljici. = said by a man
• Ja sam juče poslala pismo prijateljici. = said by a woman

The base verb is poslati, which means to send.

In this sentence, poslala is the feminine singular past participle of poslati.

A few useful forms:

• infinitive: poslati
• masculine past: poslao
• feminine past: poslala
• neuter past: poslalo
• plural: poslali / poslale

This is mainly because sam is a clitic, and clitics in Serbian usually go in the second position in the sentence or clause.

So in:

Ja sam juče poslala pismo prijateljici.

• Ja is first
• sam goes in the second position
• juče comes after that

You can also say:

• Juče sam poslala pismo prijateljici.

That is also very natural.

What sounds less natural is putting sam in a position where it breaks the normal clitic pattern.

No, Serbian word order is fairly flexible.

This sentence could appear in different orders depending on emphasis, for example:

• Ja sam juče poslala pismo prijateljici.
• Juče sam poslala pismo prijateljici.
• Pismo sam juče poslala prijateljici.
• Prijateljici sam juče poslala pismo.

These all keep roughly the same basic meaning, but the focus changes slightly.

English relies more on word order for grammar. Serbian relies more on case endings and verb forms, so word order can move around more freely.

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

However, for many neuter nouns, the nominative and accusative singular look the same.

So:

• nominative: pismo
• accusative: pismo

That is why you do not see a different ending here.

Because prijateljici is in the dative case, which is used here for the recipient of the letter.

The basic noun is:

• prijateljica = female friend

Its dative singular form is:

• prijateljici = to a female friend

So:

• pismo = the thing being sent
• prijateljici = the person receiving it

This is similar to English to my friend, except Serbian often uses the case ending instead of a separate word like to.

Yes. Prijateljici comes from prijateljica, which means a female friend.

If the friend were male, you would say:

• prijatelju = to a male friend

So:

• Ja sam juče poslala pismo prijateljici. = to a female friend
• Ja sam juče poslala pismo prijatelju. = to a male friend

Because Serbian often expresses meanings like to, for, with, and so on through case endings instead of separate prepositions.

Here, the ending -ici shows the dative meaning:

• prijateljica = female friend
• prijateljici = to the female friend

So English uses:

• to my friend

Serbian can simply use:

• prijateljici

The case ending already carries the to meaning.

Poslala comes from poslati, which is a perfective verb.

That means it presents the action as completed: the letter was sent as a finished event.

That fits well with yesterday, when you are talking about one completed action.

The imperfective partner is slati.

Very roughly:

• poslati / poslala = send, as a completed act
• slati / slala = be sending / send repeatedly / focus on the process

So Ja sam juče poslala pismo prijateljici strongly suggests one completed action: the letter got sent.

Yes, absolutely. That is very natural, and in many contexts it may even sound more natural than starting with ja.

Compare:

• Ja sam juče poslala pismo prijateljici.
  Slight emphasis on I
• Juče sam poslala pismo prijateljici.
  Slight emphasis on yesterday

Both are correct.

Because Serbian has no articles.

English distinguishes:

• a letter
• the letter

Serbian does not use separate words like a/an/the. The exact meaning depends on context.

So pismo could mean:

• a letter
• the letter

and prijateljici could mean:

• to a female friend
• to the female friend
• to my female friend, depending on context

If you want to be more specific, Serbian can add other words, such as possessives:

• mojoj prijateljici = to my female friend

The past participle changes to agree with the subject.

Examples:

• female singular: Ja sam juče poslala pismo prijateljici.
• male singular: Ja sam juče poslao pismo prijateljici.
• we, mixed group or masculine plural form: Mi smo juče poslali pismo prijateljici.
• we, all female: Mi smo juče poslale pismo prijateljici.

So Serbian past tense does not just show tense—it also shows gender and number.