Moja prijateljica sutra putuje vozom.

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

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

Compare:

• moj prijatelj = my male friend
• moja prijateljica = my female friend
• moje dete = my child

So moja is the feminine form of my here.

Prijateljica means female friend.

It is the feminine form of prijatelj = male friend / friend.

So:

• prijatelj = male friend
• prijateljica = female friend

In Serbian, this usually does not automatically mean girlfriend in the romantic sense. If you specifically want girlfriend, that is often devojka, depending on context.

In Serbian, the present tense is often used for a future event when the future meaning is already clear from context, especially from a time word like sutra.

So:

• Moja prijateljica sutra putuje vozom. = My friend is traveling / will travel by train tomorrow.

This is very natural Serbian.

You could also use the future tense:

• Moja prijateljica će sutra putovati vozom.

But the present tense version is very common and idiomatic when the event is planned or expected.

The infinitive is putovati, which means to travel.

Putuje is the 3rd person singular present form:

• ja putujem = I travel / am traveling
• ti putuješ = you travel
• on/ona/ono putuje = he/she/it travels

In this sentence, putuje matches moja prijateljica = she travels.

Because after verbs of movement meaning travel by means of transport, Serbian often uses the instrumental case.

• dictionary form: voz = train
• instrumental singular: vozom = by train

So:

• putuje vozom = travels by train

This is a very common pattern:

• ide autobusom = goes by bus
• putuje avionom = travels by plane
• ide kolima = goes by car

The ending -om is a common instrumental singular ending for masculine and neuter nouns.

Because Serbian often expresses by means of using the instrumental case alone, without a preposition.

In English, you say:

• by train

In Serbian, you usually just use the instrumental:

• vozom

So the idea of by is built into the case ending, not expressed by a separate word.

No, Serbian word order is fairly flexible.

This sentence has a very natural neutral order:

• Moja prijateljica = subject
• sutra = time
• putuje = verb
• vozom = means of transport

But other orders are also possible, depending on emphasis:

• Sutra moja prijateljica putuje vozom.
• Moja prijateljica putuje sutra vozom.
• Vozom moja prijateljica sutra putuje. — possible, but marked and less neutral

The original version sounds natural and unmarked.

Yes. Serbian often drops subject pronouns because the verb form already tells you the person and number.

For example, instead of saying Ona sutra putuje vozom, Serbian often just says:

• Sutra putuje vozom.

However, in your sentence the subject is not a pronoun but a full noun phrase:

• Moja prijateljica

So nothing needs to be added. Serbian does not need a separate word for she here.

It is in the nominative case, because it is the subject of the sentence.

• prijateljica = nominative singular
• moja also appears in nominative feminine singular to agree with it

So the structure is:

• Moja prijateljica = the subject, in nominative
• putuje = the verb
• vozom = instrumental, showing the means of travel

Putovati is imperfective.

That matters because imperfective verbs are commonly used for:

• ongoing actions
• repeated actions
• general activities
• planned future events in the present tense

Traveling is often viewed as a process, so putovati fits naturally here.

In this sentence, the focus is simply that she will be traveling / will go by train tomorrow, not on a single completed result. That is why putuje works very naturally.

You would change the words so they agree with a masculine noun:

• Moj prijatelj sutra putuje vozom.

Changes:

• moja → moj
• prijateljica → prijatelj

The rest of the sentence stays the same:

• sutra putuje vozom = is traveling by train tomorrow / will travel by train tomorrow