Moja sestra ide na posao biciklom kada je vreme lepo.

Because moja has to agree with sestra.

In Serbian, possessives like moj / moja / moje behave a lot like adjectives, so they change for gender, number, and case.

• sestra is feminine singular
• so the correct form is moja

Compare:

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

Serbian normally does not use articles.

So sestra can mean:

• a sister
• the sister
• simply sister

The exact meaning comes from context. In this sentence, moja sestra naturally means my sister.

Ide is the 3rd person singular present tense of ići = to go.

So:

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

Here moja sestra ide means my sister goes.

It can mean either, depending on context.

The Serbian present tense often covers both:

• she goes
• she is going

In this sentence, the meaning is clearly habitual because of kada je vreme lepo (when the weather is nice / whenever the weather is nice). So the natural English meaning is:

• My sister goes to work by bike when the weather is nice.

Because this sentence describes a repeated, habitual action.

Ići is imperfective, and imperfective verbs are the normal choice for:

• habits
• repeated actions
• ongoing processes

A perfective verb such as otići would suggest a single completed going/departure, not a general habit.

So:

• ide na posao = goes to work / is going to work
• otiđe na posao would not fit this habitual meaning

Na posao is the normal Serbian expression for to work in the sense of going to one’s workplace.

The preposition na is commonly used with destinations like this:

• na posao = to work
• na fakultet = to university/faculty
• na more = to the seaside

A very useful contrast is:

• na posao = to work (movement toward)
• na poslu = at work (location)

It is accusative singular, because na takes the accusative when it expresses movement toward a destination.

The basic form is:

• posao = work / job

The accusative singular is also:

• posao

That is normal for many masculine inanimate nouns in Serbian: the nominative and accusative singular often look the same.

So even though the form does not change visibly, the case is still accusative.

Biciklom is the instrumental singular of bicikl.

Serbian often uses the instrumental without a preposition to show the means by which something is done, especially with transport.

So:

• biciklom = by bicycle / by bike
• autom = by car
• autobusom = by bus
• vozom = by train

That is why ide na posao biciklom means goes to work by bike.

Yes, but it is not exactly the same.

• biciklom emphasizes the means of transport = by bike
• na biciklu literally means on the bicycle

In many situations both are possible, but biciklom is the more standard choice when you mean how someone travels.

So in this sentence, ide na posao biciklom is the most natural way to say she goes to work by bike.

They both mean when.

• kada is a bit more full/formal/neutral
• kad is shorter and very common in everyday speech

So both of these are fine:

• kada je vreme lepo
• kad je vreme lepo

The meaning is the same here.

Yes.

Vreme can mean:

• time
• weather

Context tells you which one is meant.

For example:

• Nemam vremena. = I don’t have time.
• Vreme je lepo. = The weather is nice.

In your sentence, because of lepo and the whole situation, vreme clearly means weather.

Because vreme is a neuter singular noun.

Adjectives in Serbian must agree with the noun they describe:

• lep = masculine
• lepa = feminine
• lepo = neuter

Since vreme is neuter, the correct form is:

• lepo vreme = nice weather
• vreme je lepo = the weather is nice

Yes, both are possible.

• kada je vreme lepo = when the weather is nice
• kada je lepo vreme = when the weather is nice

The difference is structural:

1. kada je vreme lepo
   This is like the weather is nice:

   • vreme = subject
   • je = is
   • lepo = predicate adjective

2. kada je lepo vreme
   This uses lepo vreme as a noun phrase:

   • literally when there is nice weather

In everyday usage, both are natural and very close in meaning.

Yes.

The combination of:

• present tense (ide)
• plus the time/condition clause kada je vreme lepo

makes it sound like something that happens regularly or whenever that condition is true.

So the meaning is not one single trip, but a repeated pattern.

Yes. Serbian word order is fairly flexible.

For example, you could also say:

• Kada je vreme lepo, moja sestra ide na posao biciklom.

This version puts the condition first and may sound especially natural in many contexts.

The original sentence is also correct. Changing word order usually changes emphasis more than basic meaning.