Moju sestru boli stomak danas.

Because moju sestru is in the accusative case.

With the verb boleti (to hurt / ache), Serbian usually treats:

• the person who feels the pain as the object
• the thing that hurts as the subject

So in Moju sestru boli stomak, my sister is the one affected, so she appears in the accusative:

• moja sestra = nominative, my sister
• moju sestru = accusative, my sister as the object

Both words change because moju has to agree with sestru in gender, number, and case.

Here stomak is actually the subject of the sentence, so it is in the nominative.

That may feel backward to an English speaker, because English usually says something like My sister’s stomach hurts or My sister has a stomachache. But Serbian uses the pattern:

X boli Y = X hurts Y

So here:

• stomak = the thing that hurts
• moju sestru = the person who is hurt

One small complication: for many masculine inanimate nouns, the nominative and accusative forms look the same. So stomak could look unchanged either way, but in this sentence its function is nominative.

Because it agrees with stomak, which is singular.

In Serbian, the verb agrees with the grammatical subject, and here the subject is stomak.

So:

• stomak boli → singular
• zubi bole → plural
• uši bole → plural

For example:

• Moju sestru boli stomak.
• Moju sestru bole uši.

The verb changes according to what hurts, not according to the person feeling the pain.

Because boleti works differently from the most common English phrasing.

A useful way to think of it is:

• English often says: My sister has a stomachache
• Serbian often says: The stomach hurts my sister

That is not the most natural English translation, but it helps explain the grammar.

This same pattern appears in many common Serbian expressions:

• Boli me glava. = My head hurts / I have a headache
• Bole ga leđa. = His back hurts
• Bole je oči. = Her eyes hurt

So the structure is very normal in Serbian.

Yes. Moju is the feminine accusative singular form of moj / moja / moje (my).

Because sestra is:

• feminine
• singular
• accusative in this sentence

the possessive has to match it:

• moja sestra = my sister (nominative)
• moju sestru = my sister (accusative)

So Serbian possessives behave more like adjectives: they change form to match the noun.

Yes, it is natural and understandable.

A Serbian speaker might also say:

• Moju sestru danas boli stomak.
• Danas moju sestru boli stomak.

All of these are fine. The differences are mostly about focus and emphasis, not basic meaning.

The version with danas at the end is perfectly possible; it can sound like today is added as the final piece of information.

Yes. Word order in Serbian is fairly flexible because case endings show the grammatical roles.

Possible versions include:

• Moju sestru boli stomak danas.
• Moju sestru danas boli stomak.
• Danas moju sestru boli stomak.

These all mean basically the same thing, but the emphasis shifts a little:

• Danas... puts today first for emphasis
• ...danas leaves today as the final point
• ...danas boli... often sounds very natural in everyday speech

Yes, that is also possible, but it is a different structure.

Compare:

• Moju sestru boli stomak.
  A very common, direct way to say she has stomach pain.

• Moja sestra ima bolove u stomaku.
  Literally My sister has pains in her stomach. This can sound a bit more descriptive or slightly more formal.

So both are correct, but boli stomak is extremely common in everyday Serbian.

In everyday Serbian, stomak often refers to the stomach / belly area in a broad, non-technical way.

So boli stomak often means stomachache or belly pain, not necessarily pain in the anatomical stomach organ only.

Related words:

• stomak = everyday stomach / belly
• trbuh = abdomen / belly
• želudac = the anatomical stomach organ

In ordinary speech, stomak is the most common word in this kind of sentence.

Yes, that is grammatically possible.

Because Serbian marks moju sestru as accusative, the sentence can still be understood even if the word order changes. But starting with the verb often sounds more marked, more dramatic, or more context-dependent.

A learner is safest using:

• Moju sestru boli stomak danas.
• Moju sestru danas boli stomak.

Those sound more neutral.

Because this sentence uses the boleti pattern, where the body part itself is the subject:

• boli stomak
• boli glava
• bole leđa

If you switch to a different construction, then a preposition may appear:

• Moja sestra ima bolove u stomaku.

So:

• with boleti → usually no preposition in this type of sentence
• with imati bolove u... → you often use u

Both are correct, just different structures.