Když otevřu okno, uvidím celý park.

Když means when in the sense of whenever / if and when.

In this sentence, Když otevřu okno, uvidím celý park. means something like:

• When I open the window, I will see the whole park
• or If I open the window, I’ll see the whole park

So když introduces a time clause: when I open the window.

One important point for English speakers: Czech often uses když where English might use either when or if, depending on context.

This is a very common Czech pattern.

The verbs are:

• otevřu = from otevřít (to open, perfective)
• uvidím = from uvidět (to see / catch sight of, perfective)

Both are perfective verbs, and in Czech, the present-tense forms of perfective verbs usually have future meaning.

So:

• otevřu literally looks like a present form, but it means I will open
• uvidím literally looks like a present form, but it means I will see

That is why the whole sentence refers to the future even though no separate future auxiliary is used.

Because otevřít belongs to a verb pattern whose 1st person singular form is -u.

Its present/future forms are:

• otevřu = I will open
• otevřeš = you will open
• otevře = he/she/it will open
• otevřeme = we will open
• otevřete = you will open
• otevřou = they will open

So otevřu is simply the correct conjugated form of otevřít.

Because they come from different verbs and aspects.

• vidět = to see (imperfective)
• uvidět = to see / catch sight of / come to see (perfective)

Compare:

• Vidím park. = I can see the park / I see the park
• Uvidím park. = I will see the park / I will catch sight of the park

In your sentence, uvidím fits because it describes the result that will happen after opening the window.

Because budu + infinitive is mainly used with imperfective verbs.

For example:

• Budu otevírat okno. = I will be opening the window / I will open the window (imperfective sense)
• Budu vidět park. would not work as the normal way to say I will see the park

But with perfective verbs, Czech normally uses the present-form endings to express future:

• otevřu
• uvidím

So Když otevřu okno, uvidím celý park. is the natural form.

Because Czech often omits subject pronouns when the verb ending already makes the subject clear.

Here:

• otevřu clearly means I will open
• uvidím clearly means I will see

So adding já is usually unnecessary.

You could say:

• Když já otevřu okno, uvidím celý park.

but that would usually add emphasis, something like:

• When I open the window...
• As for me, when I open the window...

In neutral speech, leaving out já is more natural.

Because okno is the direct object of otevřu, so it is in the accusative case. But for this noun, the accusative singular is the same as the nominative singular.

• nominative: okno
• accusative: okno

So the form does not change, even though the case has changed.

This is common with many neuter nouns in Czech.

Because park is also a direct object here, after uvidím, so it is in the accusative case.

For an inanimate masculine noun like park:

• nominative singular: park
• accusative singular: park

And the adjective agrees with it:

• nominative masculine inanimate singular: celý
• accusative masculine inanimate singular: celý

So:

• uvidím celý park = I will see the whole park

By contrast, celého parku would be genitive, which is not what this verb needs here.

Celý means whole / entire.

So celý park means:

• the whole park
• the entire park

It suggests that after opening the window, your view includes all of it, not just part of it.

The given word order is natural and neutral:

• Když otevřu okno, uvidím celý park.

But Czech word order is fairly flexible, and changes can affect emphasis.

For example:

• Uvidím celý park, když otevřu okno.
  = same basic meaning, but the main result comes first

• Když otevřu okno, celý park uvidím.
  = stronger emphasis on the whole park

• Okno když otevřu, uvidím celý park.
  = possible, but more marked and less neutral

So the original sentence is probably the best version for a learner to treat as the default pattern.

Mostly when, but in natural English translation it can feel close to if.

Czech když often covers situations where English distinguishes:

• when = time relationship
• if = condition

In this sentence, opening the window is the condition that makes seeing the park possible. So English speakers may understand it as either:

• When I open the window, I’ll see the whole park
• If I open the window, I’ll see the whole park

The Czech sentence itself is completely normal.

Because Když otevřu okno is a subordinate clause, and Czech normally separates subordinate clauses with a comma.

So the structure is:

• Když otevřu okno, = subordinate clause
• uvidím celý park. = main clause

This comma is required in standard Czech writing.

Yes, and it changes the nuance a bit.

• Když otevřu okno, uvidím celý park. = When/if I open the window, I’ll see the whole park
• Až otevřu okno, uvidím celý park. = Once / when I open the window (at that future moment), I’ll see the whole park

Až is often used for a more clearly future, one-time event.
Když is broader and very common.

So both are possible, but až can sound a little more specifically tied to a future moment.