Moj brat sada sedi, a moja prijateljica stoji kod vrata.

Because moj and moja are possessive adjectives meaning my, and they must agree with the noun in gender, number, and case.

• brat = brother → masculine singular
  so: moj brat
• prijateljica = female friend → feminine singular
  so: moja prijateljica

Other basic forms are:

• moj for masculine singular
• moja for feminine singular
• moje for neuter singular

So the change is not about the speaker; it is about the noun being described.

Brat is a masculine noun, while prijateljica is a feminine noun.

In Serbian, grammatical gender affects:

• adjectives
• possessives
• some pronouns
• past tense forms in some contexts

That is why the sentence has:

• moj brat
• moja prijateljica

Also, prijateljica specifically means a female friend. The masculine form would be prijatelj.

Because Serbian does not use a verb like to be in this sentence the way English would.

In English, you say:

• My brother is sitting
• My friend is standing

In Serbian, the present tense of verbs like sedeti (to sit) and stajati/stojati (to stand) already carries the meaning, so you simply say:

• brat sedi
• prijateljica stoji

You do not need an extra word meaning is here.

They are present-tense verb forms:

• sedi = sits / is sitting
• stoji = stands / is standing

In Serbian, the present tense often covers both:

• a simple/general meaning
• an action happening right now

So:

• Moj brat sedi can mean My brother is sitting or My brother sits, depending on context.
• Moja prijateljica stoji can mean My female friend is standing.

Here, because of sada (now), it clearly means something happening at the moment.

Sada means now. It emphasizes that the action is happening at this moment.

So:

• Moj brat sada sedi = My brother is sitting now

It is not always necessary, because sedi can already mean is sitting from context. But adding sada makes the time reference explicit.

You could say:

• Moj brat sedi = My brother is sitting / sits
• Moj brat sada sedi = My brother is sitting now

A is a very common Serbian conjunction. Here it links two clauses:

• Moj brat sada sedi
• moja prijateljica stoji kod vrata

In this sentence, a is best understood as:

• and, when contrasting two parallel facts
• sometimes almost like while

So the idea is:

• My brother is sitting, and/while my female friend is standing by the door.

It is often used when there is a mild contrast or simply a shift from one subject to another.

Compare:

• i = plain and
• a = and / while / whereas depending on context
• ali = but

Here a feels very natural because one person is doing one thing, and another person is doing something else.

Because kod means by, near, at in the sense of location next to something.

So:

• kod vrata = by the door / near the door

This is a natural way to express position.

A few useful comparisons:

• kod vrata = by the door, near the door
• na vratima = on the door
• do vrata = up to the door / as far as the door

So if someone is standing next to the door, kod vrata is the right idea.

Because vrata is one of those Serbian nouns that is normally used in a plural form even when it refers to a single object.

So:

• vrata = door but grammatically it behaves like a plural noun in many situations.

This is similar to how some nouns in other languages have unusual number behavior. Learners usually just memorize vrata as the normal word for door.

In the phrase kod vrata, the form vrata is used because of the preposition kod, which requires the genitive case. For this noun, that genitive form is also vrata.

The preposition kod takes the genitive case.

So in:

• kod vrata

the noun after kod is in the genitive.

This is useful to remember as a fixed rule:

• kod + genitive

Examples:

• kod kuće = at home
• kod prijatelja = at a friend’s place / by a friend
• kod vrata = by the door

Yes. Serbian word order is more flexible than English word order, although some versions sound more natural than others.

The given sentence:

• Moj brat sada sedi, a moja prijateljica stoji kod vrata.

is completely natural.

You could also hear:

• Moj brat sedi sada, a moja prijateljica stoji kod vrata.
• Sada moj brat sedi, a moja prijateljica stoji kod vrata.

These all mean roughly the same thing, but the focus changes a little:

• sada earlier in the sentence can emphasize now
• putting it later may sound slightly less neutral in some contexts

The original word order is a very good standard choice for learners.

The verbs agree with the subject in person and number.

For sedeti (to sit), present tense:

• ja sedim = I sit / am sitting
• ti sediš = you sit / are sitting
• on/ona/ono sedi = he/she/it sits / is sitting
• mi sedimo = we sit / are sitting
• vi sedite = you sit / are sitting
• oni/one/ona sede = they sit / are sitting

For stojati / stajati (to stand), one common present set is:

• ja stojim
• ti stojiš
• on/ona/ono stoji
• mi stojimo
• vi stojite
• oni/one/ona stoje

In your sentence:

• brat = he → sedi
• prijateljica = she → stoji

By itself, prijateljica usually means female friend. Context is important.

In everyday speech, it can sometimes also mean girlfriend, depending on the situation, but very often speakers make that clearer through context or by using other expressions if needed.

In this sentence, without special context, the most straightforward reading is simply:

• my female friend

That is one reason many learning materials translate prijateljica carefully rather than just using friend.

A simple approximate pronunciation is:

• Moj brat sada sedi, a moja prijateljica stoji kod vrata.
• moy braht SA-da SE-di, a MO-ya pri-ya-TE-yi-tsa STO-yi kod VRA-ta

A few useful notes:

• j in Serbian sounds like English y in yes
• moj sounds roughly like moy
• moja sounds like MO-ya
• c sounds like ts
• č and ć are different Serbian sounds, but neither appears here
• jica in prijateljica is pronounced with a y sound before the next vowel

You do not need perfect pronunciation immediately, but recognizing j = y helps a lot.