Ja vozim bicikl kada je vreme lepo.

Yes, you can omit ja.

Serbian often leaves out subject pronouns because the verb ending already shows who is doing the action:

• vozim = I ride / I drive

So:

• Ja vozim bicikl kada je vreme lepo.
• Vozim bicikl kada je vreme lepo.

Both are correct.

Including ja usually adds emphasis, contrast, or clarity, for example:

• Ja vozim bicikl, a on trči.
  I ride a bike, and he runs.

Without special emphasis, learners will often hear the version without ja more often.

Vozim is the 1st person singular present tense of voziti.

Depending on context, voziti can mean:

• to drive
• to ride
• to operate a vehicle
• sometimes to take/transport someone by vehicle

In this sentence, with bicikl, it means:

• I ride a bike

So:

• vozim bicikl = I ride a bicycle

English separates drive and ride, but Serbian often uses voziti for both, depending on what the vehicle is.

Because bicikl is the direct object of vozim, so it goes in the accusative case.

However, bicikl is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: bicikl
• accusative: bicikl

That is why you do not see a change here.

Compare that with a masculine animate noun, where the accusative usually changes:

• Vidim čoveka = I see a man

But:

• Vozim bicikl = I ride a bicycle

Because lepo agrees with vreme.

The noun vreme is neuter singular, so the adjective must also be neuter singular:

• masculine: lep
• feminine: lepa
• neuter: lepo

So:

• lep dan = a nice day
• lepa kuća = a nice house
• lepo vreme = nice weather

That is standard adjective agreement in Serbian.

Here it means weather.

The word vreme can mean both:

• time
• weather

In this sentence, the phrase lepo vreme clearly means:

• nice weather

This is a very common Serbian expression.

So:

• Koliko je vremena? can relate to time
• Lepo je vreme. means The weather is nice.

Context tells you which meaning is intended.

Because Serbian has special placement rules for short unstressed words called clitics, and je is one of them.

In a clause like this, je usually comes in the second position of the clause, not necessarily directly after the subject in the English way.

So Serbian prefers:

• kada je vreme lepo

not:

• kada vreme je lepo

This is one of the most important word-order patterns in Serbian.

You can think of it roughly like this:

• kada = first element
• je = clitic in second position
• vreme lepo = the rest

This is why Serbian word order may look unusual to English speakers.

Yes, absolutely.

Both are natural:

• kada je vreme lepo
• kada je lepo vreme

The second one is often especially common because lepo vreme is a strong fixed phrase meaning nice weather.

So these are both fine:

• Vozim bicikl kada je vreme lepo.
• Vozim bicikl kada je lepo vreme.

The meaning is basically the same.

Very little in meaning.

Both mean when.

• kada is the full form
• kad is the shorter, very common form in everyday speech

So these both work:

• Vozim bicikl kada je vreme lepo.
• Vozim bicikl kad je vreme lepo.

In conversation, learners will often hear kad a lot.

Normally, it means a habitual action:

• I ride a bike when the weather is nice

Serbian present tense often covers both:

• simple present
• present continuous, depending on context

But in this sentence, with kada je vreme lepo, the most natural interpretation is a repeated habit or general truth.

If you wanted to strongly emphasize what is happening right now, Serbian would usually rely on context or different phrasing rather than a special continuous tense like English.

Your sentence is correct, but many speakers would more naturally say:

• Kad je lepo vreme, vozim bicikl.

This version puts the when-clause first, which is very common.

So all of these are good:

• Ja vozim bicikl kada je vreme lepo.
• Vozim bicikl kada je vreme lepo.
• Kad je lepo vreme, vozim bicikl.

The main differences are emphasis and style, not basic meaning.

Because Serbian does not have articles like English a/an/the.

So Serbian simply says:

• vozim bicikl = I ride a bike / the bike
• vreme je lepo = the weather is nice

Whether English should use a, the, or no article at all depends on context and translation.

This is very normal in Serbian and in many other Slavic languages.

You may hear similar expressions, but vozim bicikl is the clearest and most standard way to say I ride a bike.

A few related patterns are:

• vozim bicikl = I ride a bike
• idem biciklom = I go by bike
• vozim se = I am riding / I am being transported

For a beginner, it is safest to learn:

• voziti bicikl = to ride a bike

That is the direct and standard phrase to use in sentences like this.