Bilo da idemo autobusom ili pješice, u grad ćemo doći na vrijeme.

Bilo da ... ili ... is a common Croatian pattern meaning whether ... or ....

So:

• Bilo da idemo autobusom ili pješice = Whether we go by bus or on foot

It introduces two possible situations, and the main clause says the result is the same in either case.

You can think of bilo da as an idiomatic connector rather than trying to translate each word separately every time.

After bilo da, Croatian often uses the present tense even when English would naturally use a future idea.

So:

• bilo da idemo literally looks like whether we go
• but in context it can refer to a future possibility: whether we go / whether we end up going

This is normal Croatian usage. The future meaning is understood from the whole sentence, especially because the main clause has a future form:

• u grad ćemo doći na vrijeme = we will arrive in town on time

So the sentence does not need ići ćemo in the bilo da clause.

Autobusom is the instrumental singular of autobus.

Croatian often uses the instrumental case to express the means of transport:

• autobusom = by bus
• vlakom = by train
• autom = by car

So:

• idemo autobusom = we are going by bus

This is a very common pattern.

Pješice is an adverb, meaning on foot.

So:

• ići pješice = to go on foot
• doći pješice = to arrive on foot

It is simply the normal Croatian word used for this idea. You do not usually replace it with a noun in a case form.

You may also hear pješke, which means the same thing. Pješice is very common and standard.

This is a very important Croatian pattern:

• u + accusative = movement into a place
• u + locative = being in a place

So:

• u grad = into the city / to town
• u gradu = in the city / in town

Because doći expresses movement toward a destination, Croatian uses u grad.

Compare:

• Idemo u grad. = We are going to town.
• Mi smo u gradu. = We are in town.

Both are possible in Croatian, but word order affects where the clitic ćemo goes.

In this sentence, the phrase u grad comes first in the main clause:

• u grad ćemo doći na vrijeme

Croatian clitics such as ću, ćeš, će, ćemo, ćete usually appear in the second position of the clause. Since u grad is placed first for emphasis or style, ćemo comes right after it.

So this is natural:

• U grad ćemo doći na vrijeme.

You could also say:

• Doći ćemo u grad na vrijeme.

That is also correct, just with a different word order.

In Croatian future tense, when the auxiliary comes after the infinitive, the final -i of the infinitive is often dropped in writing:

• doći ćemo
• ići ćemo

But when the auxiliary comes before the infinitive, the infinitive stays in its normal full form:

• ćemo doći
• ćemo ići

So in your sentence:

• u grad ćemo doći na vrijeme

the form doći remains unchanged because ćemo comes before it.

Na vrijeme means on time or in time, depending on context.

In this sentence, it clearly means:

• on time
• not late

This is a fixed expression:

• Došao je na vrijeme. = He arrived on time.
• Moramo stići na vrijeme. = We have to arrive on time.

Do not confuse it with expressions that literally mean something like in the time; na vrijeme is the normal phrase for punctuality.

Yes, it is a standard structure for expressing alternatives.

In your sentence:

• Bilo da idemo autobusom ili pješice ...

Croatian speakers use this pattern naturally to mean whether ... or ...

You may also encounter related patterns in Croatian, but for a learner, it is best to remember this as one chunk:

• bilo da X ili Y

That will help you produce natural sentences more easily.

The first part of the sentence is a subordinate clause:

• Bilo da idemo autobusom ili pješice

The second part is the main clause:

• u grad ćemo doći na vrijeme

Croatian normally separates this kind of introductory subordinate clause with a comma, just as English often does:

• Whether we go by bus or on foot, we’ll arrive on time.

So the comma is expected here.

Idemo is first person plural of ići in the present tense:

• I go = idem
• you go = ideš
• he/she goes = ide
• we go = idemo
• you (plural) go = idete
• they go = idu

So idemo means we go / we are going.

That matches the future verb in the main clause:

• ćemo doći = we will come / we will arrive

Both parts refer to we.

Both verbs can often relate to arriving, but they are not always identical.

• doći basically means to come
• stići often emphasizes to arrive / to reach / to make it

In this sentence, doći u grad is perfectly natural and means to come to town / arrive in town.

You could also hear:

• u grad ćemo stići na vrijeme

That would also be natural, with slightly more focus on the idea of successfully arriving. But doći is completely normal here.

Yes. Croatian word order is fairly flexible, although some versions sound more natural in certain contexts.

For example, these are possible:

• Bilo da idemo autobusom ili pješice, u grad ćemo doći na vrijeme.
• Bilo da idemo autobusom ili pješice, doći ćemo u grad na vrijeme.

The meaning stays basically the same. The version with u grad ćemo doći gives a little prominence to the destination u grad.

This flexibility is very common in Croatian, especially because clitics like ćemo follow second-position rules.