Danas je poštarka donela mali paket za komšinicu.

Because je is a clitic in Serbian. Clitics are short unstressed words that usually go in the second position in the clause.

So in:

Danas je poštarka donela mali paket za komšinicu.

the first element is Danas, and the clitic je comes right after it.

This is very typical in Serbian:

• Danas je došla.
• Poštarka je danas došla.
• Moj komšija je juče otišao.

English speakers often expect a fixed subject-verb order, but Serbian word order is more flexible, while clitics still follow this special placement rule.

Je donela is the perfect tense in Serbian, used very often for past actions.

It is formed with:

• the auxiliary je (is/has-type helper verb, from biti = to be)
• the past participle: donela

So:

• je donela = brought / has brought

This is the normal way to talk about completed past events in everyday Serbian.

Compare:

• on je doneo = he brought
• ona je donela = she brought
• oni su doneli = they brought

Because the past participle agrees with the subject in gender and number.

The subject here is poštarka, which is:

• feminine
• singular

So the correct form is donela.

Compare:

• poštar je doneo = the mailman brought
• poštarka je donela = the mailwoman brought
• poštari su doneli = the mail carriers brought

This agreement is very important in Serbian past-tense forms.

Poštarka is the feminine form meaning female postal worker / mail carrier / postwoman.

The masculine form is:

• poštar = mailman / postman

Serbian often has masculine and feminine forms for people’s roles or professions:

• učitelj / učiteljica
• glumac / glumica
• komšija / komšinica

Since this sentence uses poštarka, the person is specifically female.

Because za here means for, and after za in this meaning, Serbian uses the accusative case.

The basic dictionary form is:

• komšinica = neighbor (female)

But in the accusative singular, it becomes:

• komšinicu

So:

• za komšinicu = for the neighbor

This is a very common pattern for feminine nouns ending in -a:

• sestra → sestru
• učiteljica → učiteljicu
• komšinica → komšinicu

Mali paket is the direct object, so it is in the accusative case.

However, paket is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative looks the same as the nominative.

So:

• nominative: mali paket
• accusative: mali paket

That is why you do not see a change.

Compare this with a masculine animate noun, where the accusative usually does change:

• nominative: dobar čovek
• accusative: dobrog čoveka

So paket stays the same because it is masculine inanimate.

Because adjectives in Serbian must agree with the noun they describe in:

• gender
• number
• case

The noun paket is:

• masculine
• singular
• here accusative, but masculine inanimate accusative has the same form as nominative

So the adjective is:

• mali

Compare:

• mali paket = small package
• mala kutija = small box
• malo pismo = small letter

The adjective changes depending on the noun.

Here za means for in the sense of intended for someone.

So:

• paket za komšinicu = a package for the neighbor

This describes who the package is meant for.

Serbian can also express a related idea in another way:

• Poštarka je donela komšinici mali paket.

That means something like:

• The mail carrier brought the neighbor a small package.

So the difference is roughly:

• paket za komšinicu = a package intended for the neighbor
• donela komšinici paket = brought a package to the neighbor

In real life, these can overlap in meaning, but the structure is different.

Yes. Serbian word order is fairly flexible because cases show the grammatical roles.

Your sentence:

• Danas je poštarka donela mali paket za komšinicu.

A different order could be:

• Poštarka je danas donela mali paket za komšinicu.

Both are correct. The difference is mostly emphasis:

• starting with Danas highlights today
• starting with Poštarka highlights the mail carrier

What usually stays important is the placement of clitics like je, which tend to stay in second position.

Because Serbian does not have articles like English a/an/the.

So mali paket can mean:

• a small package
• the small package

The exact meaning depends on context.

If Serbian speakers want to make it clearer, they can add other words, for example:

• jedan mali paket = a small package
• taj mali paket = that/the small package

But very often, no article is needed.

Yes. Donela comes from the verb doneti = to bring.

This verb is perfective, which means it presents the action as completed.

In your sentence, that fits well because the action is a finished event:

• the mail carrier brought the package

A related imperfective verb is:

• donositi = to be bringing / to bring repeatedly / habitually

Compare:

• Danas je poštarka donela paket. = completed action today
• Poštarka je često donosila pakete. = she used to bring packages often

This perfective/imperfective contrast is a major part of Serbian verbs.

Possibly, yes. The main difference you might notice is in the past participle of doneti.

In this sentence you have:

• donela

In some varieties, especially ijekavian-based forms, you may see:

• donijela

Both correspond to the same verb and meaning. A learner may encounter both depending on region and standard variety.

So you may see:

• Danas je poštarka donela mali paket za komšinicu.
• Danas je poštarka donijela mali paket za komšinicu.

Both are normal, depending on the variety being used.

Because the noun itself names the subject, and Serbian does not need an extra subject pronoun like she.

Here the subject is explicitly:

• poštarka

So there is no need to say a separate word for she.

Serbian often leaves pronouns out when they are unnecessary:

• Došla je. = She came / He came (context decides)
• Poštarka je došla. = The mail carrier came

Also, the past participle helps, because donela shows a feminine singular subject, which supports poštarka.