Když bude na pěšině bláto, dorazíme později.

It can be understood as either, depending on context.

In this sentence, když is very natural for a real-life condition:

• Když bude na pěšině bláto, dorazíme později.

In English, you might often translate it as if:

• If there is mud on the path, we'll arrive later.

But Czech když often overlaps with English when/if in this kind of sentence. It does not always force a strict distinction.

Because Czech normally uses the actual future tense in a future-time subordinate clause.

So Czech says:

• když bude... = literally when/if there will be...

That is completely normal Czech.

English works differently:

• If there is mud...
• When there is mud...

So this is a good example of a difference between Czech and English grammar, not a strange or exceptional Czech sentence.

Because na here means on or on the surface/in the area of, and with a static location it takes the locative case.

The noun is:

• pěšina = path, trail

Its locative singular form is:

• pěšině

So:

• na pěšině = on the path

Compare:

• na pěšině = on the path, located there
• na pěšinu = onto the path, movement toward it

Here bláto is nominative singular.

It is the thing that will be present:

• Na pěšině bude bláto. = There will be mud on the path.

Czech often uses this pattern for existence or presence:

• Je tam voda.
• Na silnici je led.
• Na pěšině bude bláto.

So bláto stays in its dictionary form because it functions as the grammatical subject.

Because dorazit is a perfective verb.

In Czech, perfective verbs use present-tense forms to express future meaning:

• dorazím = I will arrive
• dorazíme = we will arrive

So:

• dorazíme později = we will arrive later

It does not mean an ongoing present action.

Because Czech usually leaves out subject pronouns when they are already clear from the verb ending.

In:

• dorazíme

the ending -íme already tells you it means we.

So:

• dorazíme = we will arrive

You could add my for emphasis:

• My dorazíme později.

But without emphasis, leaving it out is more normal.

Because později means later, while pozdě means late.

• dorazíme pozdě = we will arrive late
• dorazíme později = we will arrive later

In this sentence, the idea is comparative: because of the mud, arrival will happen later than expected or later than it otherwise would.

Yes.

In Czech, a subordinate clause introduced by když is separated by a comma.

So this is correct:

• Když bude na pěšině bláto, dorazíme později.

And if you reverse the order, you still use a comma:

• Dorazíme později, když bude na pěšině bláto.

Yes. Czech word order is fairly flexible.

These are all possible:

• Když bude na pěšině bláto, dorazíme později.
• Dorazíme později, když bude na pěšině bláto.
• Když na pěšině bude bláto, dorazíme později.

The basic meaning stays the same, but the emphasis can shift a little. The original version is a very natural neutral sentence.

Because když is very common and natural in everyday Czech for this kind of condition.

Roughly:

• když = when / if
• jestli = if, whether
• pokud = if, provided that, often more formal

So:

• Jestli bude na pěšině bláto, dorazíme později. is also possible.
• Pokud bude na pěšině bláto, dorazíme později. is possible too, but sounds a bit more formal.

The original sentence sounds natural and conversational.

Because až usually means something like when/once, often with the idea that the event is expected to happen.

• až bude... = when it is / once it becomes
• když bude... = if/when it is

In this sentence, když is better if the speaker means a condition:

• If there’s mud on the path, we’ll arrive later.

Using až would make it sound more like the muddy condition is expected or taken for granted.

You know from context, because Czech has no articles.

So:

• na pěšině can mean on a path or on the path

English has to choose, but Czech does not mark that difference with a word like a or the. The surrounding situation tells you which one is meant.

Yes: it is a very common Czech way to express that something is present somewhere.

Literally, the structure is:

• will be + location + thing

So:

• bude na pěšině bláto = there will be mud on the path

You could also encounter:

• Na pěšině bude bláto.

That version may sound slightly more natural to some learners because the location comes first, but both word orders are normal.