Když bude pršet, pojedu autobusem.

It can feel like both, depending on context.

In this sentence, když bude pršet, pojedu autobusem, když introduces a time/condition clause. In natural English, this is often translated as:

• When it rains, I’ll go by bus
• If it rains, I’ll go by bus

Czech když often covers situations where English chooses either when or if. Here it means something like in the event that / whenever it turns out that it rains.

A useful contrast:

• když = when / whenever / if in many real-life situations
• jestli / pokud = more clearly if / whether

So this sentence is perfectly natural with když.

Because the sentence refers to the future.

• prší = it is raining
• bude pršet = it will rain / it is going to rain

In Czech, the verb pršet is an imperfective verb, so its future is made with být + infinitive:

• bude pršet = it will rain

That is similar to:

• budu dělat = I will do / I will be doing

So když bude pršet means when/if it rains in the future.

This is a very common question for English speakers.

English usually avoids will after when in this kind of sentence:

• When it rains, I’ll go by bus
• not normally When it will rain...

Czech works differently. If the action is in the future, Czech usually marks it as future:

• Když bude pršet...

So Czech is being grammatically consistent about future time, even inside the když clause.

This is normal and correct Czech.

Because Czech does not need a dummy subject like English it.

In English, weather expressions usually require it:

• it is raining
• it will rain

In Czech, weather verbs are often used without any subject:

• prší = it is raining
• bude pršet = it will rain

So there is no Czech equivalent of the English dummy it here.

Because pojedu is the normal future form of the verb jet.

Jet means to go by vehicle / to ride / to travel. It is a verb of motion, and its future is usually expressed with a simple future form:

• jedu = I am going / I’m riding
• pojedu = I will go / I’ll ride / I’ll travel

So:

• pojedu autobusem = I’ll go by bus

Using budu jet is generally not the normal choice here.

For many motion verbs, Czech often uses a simple future form rather than budu + infinitive.

Here pojedu means something like:

• I’ll go
• I’ll travel
• I’ll ride
• I’ll take

Because it is followed by autobusem, the full meaning is I’ll go by bus or I’ll take the bus.

So pojedu is not just any kind of go. It specifically suggests movement by some vehicle.

Compare:

• půjdu = I’ll go on foot
• pojedu = I’ll go by vehicle

Because Czech commonly uses the instrumental after verbs of travel to express the means of transport.

So:

• autobus = bus
• autobusem = by bus

This is the normal pattern:

• jet autobusem = to go by bus
• jet vlakem = to go by train
• jet autem = to go by car
• letět letadlem = to go/fly by plane

For a learner, it is useful to remember this as a set phrase: jet + instrumental = go by means of transport

In this sentence, it means by bus.

• pojedu autobusem = I’ll go by bus / I’ll take the bus

It focuses on the mode of transport, not your physical location.

If you want to stress being on the bus as a place, Czech would usually use a different structure, for example with a preposition:

• v autobuse = in/on the bus

But here the point is transportation method, so the instrumental autobusem is exactly right.

Because Czech usually drops subject pronouns when they are clear from the verb ending.

• pojedu already means I will go
• the ending -u shows first person singular

So já is not necessary.

You can add já for emphasis or contrast:

• Já pojedu autobusem, ale ty můžeš jít pěšky. = I’ll go by bus, but you can go on foot.

Without emphasis, leaving it out is more natural.

Because the sentence contains a subordinate clause introduced by když.

Structure:

• Když bude pršet, = subordinate clause
• pojedu autobusem. = main clause

Czech normally separates this kind of clause with a comma.

So the comma is required here.

Yes, absolutely.

Both are correct:

• Když bude pršet, pojedu autobusem.
• Pojedu autobusem, když bude pršet.

The meaning stays basically the same.

The difference is mainly in focus:

• starting with Když bude pršet... puts the condition first
• starting with Pojedu autobusem... puts the main action first

In both versions, Czech still uses a comma.

In this exact form, it most naturally refers to a future situation:

• If/when it rains, I’ll go by bus

It sounds like a decision about an upcoming case.

For a more habitual or general meaning, Czech often uses present tense:

• Když prší, jezdím autobusem. = When it rains, I go by bus.

So:

• když bude pršet, pojedu... = future specific situation
• když prší, jezdím... = usual habit

Yes, but the nuance changes slightly.

• Když bude pršet, pojedu autobusem.
  = If/when it rains, I’ll go by bus.
  This is very natural and common.

• Jestli bude pršet, pojedu autobusem.
  = If it rains, I’ll go by bus.
  This sounds a bit more explicitly conditional.

So both are possible, but když is very natural in everyday Czech for this kind of sentence.

Not if you want the same meaning.

• pojedu autobusem = I’ll go by bus once, in that situation
• budu jezdit autobusem = I’ll be going by bus / I’ll go by bus regularly

The verb jezdit usually suggests repeated, habitual, or back-and-forth travel.

So in your sentence, pojedu is the right choice because it refers to one future trip or one future occasion.