Moja sestra sutra mora da ide na posao.

Moja is the feminine singular form of the possessive adjective meaning my.

It changes to match the noun it describes:

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

Since sestra (sister) is a feminine singular noun, the correct form is moja sestra.

Examples:

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

Here sestra is the subject of the sentence, so it stays in the nominative case.

The sentence is about what my sister must do, so moja sestra is the one performing the action.

You would see another form like sestru if sister were the direct object, for example:

• Vidim sestru. = I see my sister.

But in:

• Moja sestra sutra mora da ide na posao.

moja sestra is the subject, so nominative is correct.

Sutra means tomorrow. It is an adverb of time, telling you when the action happens.

So:

• Moja sestra sutra mora da ide na posao. means that the obligation applies tomorrow.

It can often move around in the sentence because Serbian word order is fairly flexible:

• Moja sestra sutra mora da ide na posao.
• Sutra moja sestra mora da ide na posao.
• Moja sestra mora sutra da ide na posao.

All are grammatical, though the emphasis may shift slightly.

Mora is the 3rd person singular present tense of the verb morati, which means must / have to.

Because the subject is moja sestra = my sister = she, the verb must be in the he/she/it form:

• ja moram = I must
• ti moraš = you must
• on/ona/ono mora = he/she/it must
• mi moramo = we must
• vi morate = you must
• oni/one/ona moraju = they must

So moja sestra mora = my sister must.

In Serbian, after verbs like morati, it is very common to use da + present tense instead of the infinitive.

So:

• mora da ide = must go

Literally, it looks like:

• must that she goes

But in natural English, you translate it simply as must go.

This da + present structure is extremely common in modern Serbian:

• Moram da radim. = I have to work.
• Hoću da idem. = I want to go.
• Može da dođe. = He/She can come.

You may also sometimes encounter infinitives in Serbian, but da + present is the normal pattern in everyday speech.

Ide is the 3rd person singular present tense of ići, which means to go.

Since the understood subject is still moja sestra (she), the verb takes the she form:

• ja idem = I go
• ti ideš = you go
• on/ona ide = he/she goes

So:

• mora da ide = she must go

Even though English uses the bare infinitive after must (must go), Serbian uses the present tense after da:

• da ide = literally that she goes, but naturally to go / go

Na posao means to work or more literally to the workplace/job.

Here na expresses movement toward a destination, so posao is in the accusative case.

The noun is:

• nominative: posao = work/job
• accusative: posao

In this noun, nominative and accusative happen to look the same, but the case is still accusative because the phrase answers where to?

Compare:

• Ide na posao. = He/She is going to work. → movement, so accusative
• Je na poslu. = He/She is at work. → location, so locative

That contrast is very useful:

• na posao = to work
• na poslu = at work

Prepositions in Serbian often have to be learned together with the noun or expression. For going to work, Serbian normally uses na posao.

So:

• ići na posao = to go to work
• biti na poslu = to be at work

Even though English uses to, Serbian chooses na in this expression. It is just the standard idiomatic pattern.

Some places use u:

• ići u školu = go to school
• ići u grad = go to town/the city

But for work in the sense of one's workplace/job, Serbian normally says na posao.

Grammatically, mora and ide are present-tense forms, but because of sutra (tomorrow), the whole sentence refers to a future situation.

This is very natural in Serbian. Present-tense forms are often used when another word already makes the time clear.

So:

• Moja sestra sutra mora da ide na posao. means My sister has to go to work tomorrow, even though the verbs are present forms.

The idea is:

• present form + future time word = future meaning

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

The most neutral version here is:

• Moja sestra sutra mora da ide na posao.

But these are also possible:

• Sutra moja sestra mora da ide na posao.
• Moja sestra mora sutra da ide na posao.
• Na posao moja sestra sutra mora da ide.
  This one is more marked and would give special emphasis.

The basic meaning stays the same, but changing the order can shift emphasis:

• sutra first emphasizes when
• na posao first emphasizes where
• moja sestra first emphasizes who

Yes, it could, but the meaning would shift slightly.

• Moja sestra = my sister
• Sestra = the sister / sister, depending on context

Serbian often omits possessive words when the relationship is already clear from context, but if you specifically want to say my sister, then moja sestra is the clearest and most direct choice.

So:

• Moja sestra sutra mora da ide na posao. = My sister has to go to work tomorrow.
• Sestra sutra mora da ide na posao. = Sister / The sister has to go to work tomorrow.
  This would usually need context to sound natural in English.

Yes. Morati can often be translated as either must or have to, depending on context and style.

So this sentence could be understood as:

• My sister must go to work tomorrow.
• My sister has to go to work tomorrow.

In many everyday situations, has to sounds more natural in English, while must can sound stronger or more formal. Serbian mora itself does not force exactly one of those English choices; both are usually possible.