Nadamo se da će vreme biti dobro, ali ona se brine zbog voza.

Because the verb is nadati se = to hope. It is a reflexive verb, so it normally comes with se.

• ja se nadam = I hope
• mi se nadamo = we hope

So nadamo se is the normal way to say we hope.
Without se, it would sound incomplete or incorrect in standard Serbian.

Nadamo is the 1st person plural present tense form of nadati se.

It corresponds to we hope.

The full form here is:

• mi se nadamo = we hope

In Serbian, the subject pronoun mi is often omitted because the verb ending already shows who the subject is.

After verbs like nadati se (to hope), Serbian very often uses a da-clause.

So:

• Nadamo se da će vreme biti dobro = We hope that the weather will be good.

This is very natural in Serbian. English often uses either:

• we hope that...
• or we hope to...

But in this sentence, Serbian uses a full clause with da + future tense:

• da će... biti...

That is the standard way to express that ... will be ...

Će is the auxiliary used to form the future tense.

In da će vreme biti dobro:

• će marks the future
• biti is the infinitive to be

So literally, the structure is something like:

• that will weather be good

Natural English:

• that the weather will be good

This is a very common Serbian future construction.

Because vreme is a neuter noun.

In Serbian, adjectives must agree with the noun in:

• gender
• number
• case

Here:

• vreme = neuter singular
• so the adjective must also be neuter singular: dobro

Compare:

• dobar voz = a good train (masculine)
• dobra knjiga = a good book (feminine)
• dobro vreme = good weather (neuter)

It can mean both, depending on context.

• vreme = time
• vreme = weather

In this sentence, because of biti dobro (to be good), it clearly means weather:

• vreme biti dobro = the weather to be good

This is very common in Serbian, so context tells you which meaning is intended.

Yes. Brinuti se means to worry or to be worried, and it is commonly used with se.

So:

• ona se brine = she worries / she is worried

Like nadati se, this verb normally appears with se in this meaning.

You may also see brinuti without se, but that usually means to take care of or to concern someone, depending on context. In this sentence, the meaning is clearly to worry, so se is needed.

Because the preposition zbog requires the genitive case.

The noun voz (train) changes in the genitive singular:

• nominative: voz
• genitive: voza

So:

• zbog voza = because of the train / about the train

This is a standard case pattern:

• zbog + genitive

It can suggest either of these ideas in English, depending on context:

• because of the train
• about the train
• due to the train

In this sentence, with ona se brine, it most naturally means:

• she is worried about the train or
• she is worried because of the train

So zbog often expresses the cause or reason for the worry.

That is because se and će are clitics in Serbian. Clitics are short unstressed words that tend to appear near the beginning of their clause, usually in second position.

So in:

• Nadamo se da će vreme biti dobro
• ona se brine

the clitics are placed in their normal Serbian positions.

English speakers often expect them to sit directly next to the verb in a more fixed way, but Serbian word order follows clitic rules, not English-style stress patterns.

Usually, no—not in this exact structure.

Serbian often omits subject pronouns, but here se is a clitic and cannot simply stand in a way that leaves the clause awkward or unclear. The verb form brine by itself does not clearly show the person as strongly as some other forms do, so keeping ona is natural and clear:

• ali ona se brine zbog voza

If the subject were already very clear from context, Serbian might rephrase the sentence, but in this sentence ona helps identify she and makes the clause sound complete and natural.

It contains both.

• Nadamo se = present tense (we hope)
• ona se brine = present tense (she worries / is worried)
• će ... biti = future tense (will be)

So the sentence means:

• right now, we hope
• right now, she is worried
• the weather is something expected in the future

That mix of present and future is completely normal.