Moja sestra se tušira u kupatilu, a ja se oblačim u sobi.

Because moja has to agree with sestra in gender, number, and case.

• sestra is feminine singular
• so the possessive adjective must also be feminine singular: moja

Compare:

• moj brat = my brother
• moja sestra = my sister
• moje dete = my child

So moja sestra is the correct agreement.

Se is a reflexive particle. In verbs like tuširati se and oblačiti se, it is part of the verb itself.

So:

• tuširati se = to shower
• oblačiti se = to get dressed / to dress oneself

In English, we usually do not translate se literally as oneself every time. For example:

• Tuširam se. = I’m showering.
• Oblačim se. = I’m getting dressed.

So here, se is required because these are reflexive verbs in Serbian.

Because se is a clitic, and clitics in Serbian usually go in the second position in the clause.

That is why you get:

• Moja sestra se tušira...
• ja se oblačim...

The important idea is that se usually cannot just go anywhere. It tends to come early in the sentence, after the first word or phrase.

This placement is very natural in Serbian and is something learners gradually get used to.

Because after u, when you mean location and not movement, Serbian uses the locative case.

Here the meaning is:

• she is showering in the bathroom
• I am getting dressed in the room

So these are static locations, which is why we use:

• u kupatilu
• u sobi

If there were movement into a place, Serbian would normally use the accusative instead:

• Idem u kupatilo. = I’m going into the bathroom.
• Ulazim u sobu. = I’m entering the room.

So:

• where? → locative
• where to? → accusative

A often connects two clauses that are contrasted or set side by side, but not as strongly as ali.

In this sentence:

• Moja sestra se tušira u kupatilu, a ja se oblačim u sobi.

the idea is something like:

• My sister is showering in the bathroom, while I am getting dressed in the room.

So a works well because it shows a mild contrast between what she is doing and what I am doing.

Very roughly:

• i = and
• a = and / while / whereas
• ali = but

Using a here is very natural.

Because the verbs are conjugated for different persons:

• moja sestra = she → 3rd person singular
• ja = I → 1st person singular

So:

• (ona) se tušira = she is showering
• (ja) se oblačim = I am getting dressed

The endings tell you who is doing the action.

Here are the full present-tense forms:

tuširati se

• ja se tuširam
• ti se tuširaš
• on/ona se tušira
• mi se tuširamo
• vi se tuširate
• oni/one se tuširaju

oblačiti se

• ja se oblačim
• ti se oblačiš
• on/ona se oblači
• mi se oblačimo
• vi se oblačite
• oni/one se oblače

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

So you could say:

• Moja sestra se tušira u kupatilu, a oblačim se u sobi.

This is grammatically possible.

However, ja is included here for contrast:

• my sister is doing one thing,
• I am doing another.

So a ja sounds very natural because it highlights the difference between the two people.

The dictionary forms are:

• tuširati se
• oblačiti se

These are both imperfective verbs, which is exactly what you want here, because the sentence describes an ongoing action or a situation in progress.

• tušira se = is showering
• oblačim se = am getting dressed

A perfective verb would usually focus more on the action being completed rather than in progress. For example, obući se means to get dressed / to put on clothes completely, not the ongoing process in the same way.

So the imperfective forms fit this sentence well.

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

So:

• u kupatilu can mean in the bathroom
• u sobi can mean in the room

The exact meaning depends on the context.

English requires articles, but Serbian usually expresses definiteness through context, word order, or general situation rather than separate words like the.

Because different nouns form the locative singular differently depending on their noun class and gender.

• kupatilo is a neuter noun ending in -o

  • nominative: kupatilo
  • locative: kupatilu

• soba is a feminine noun ending in -a

  • nominative: soba
  • locative: sobi

So the changes are part of normal noun declension.

A useful pattern is:

• many -a feminine nouns → locative -i
• many neuter nouns in -o / -e → locative -u

Not completely. Serbian word order is fairly flexible, but some orders sound more natural than others, especially because of clitics like se.

The given sentence:

• Moja sestra se tušira u kupatilu, a ja se oblačim u sobi.

is a very natural, neutral way to say it.

You can move some parts for emphasis, for example:

• U kupatilu se moja sestra tušira...
• U sobi se oblačim...

But these versions sound more marked and depend on what you want to emphasize.

So the default sentence is good to learn as the most natural everyday pattern.