Až přijdu do kuchyně, budu vařit vodu na čaj.

Here až means when, once, or as soon as in reference to a future event.

So Až přijdu do kuchyně... means something like:

• When I get to the kitchen...
• Once I come into the kitchen...
• As soon as I get to the kitchen...

In Czech, až is very commonly used to introduce a future time clause.

This is a very common point of confusion.

In Czech, after words like až, a verb that looks like the present tense is often used to express a future action, especially if the verb is perfective.

• přijít = perfective verb, meaning to arrive / to come
• přijdu = formally present tense, but with a perfective verb it refers to the future: I will come / I will arrive

So:

• Až přijdu... = When I arrive...

This is different from English, but it is normal Czech grammar.

Because do requires the genitive case, and kuchyně is the genitive singular form of kuchyň.

• nominative: kuchyň = kitchen
• genitive: kuchyně

So:

• do kuchyně = into/to the kitchen

This is part of a very important Czech pattern:

• do + genitive = movement into/to an enclosed place

For example:

• do domu = into the house
• do školy = to school
• do kuchyně = into the kitchen

This is about aspect.

Czech often distinguishes between:

• imperfective: focusing on the process or activity
• perfective: focusing on completion

Here:

• vařit = imperfective, to boil / to cook
• budu vařit = future of an imperfective verb, meaning I will be boiling / I will boil
• uvařit = perfective, to boil/cook completely, to prepare successfully

So the sentence with budu vařit focuses more on the activity itself:

• Až přijdu do kuchyně, budu vařit vodu na čaj. = When I get to the kitchen, I’ll boil / be boiling water for tea.

If you said Až přijdu do kuchyně, uvařím vodu na čaj, the focus would be more on completing the action.

Both can make sense, but they are not exactly identical in nuance.

Yes, in Czech vařit is used for both cooking and boiling.

So vařit vodu means:

• to boil water
• literally something like to cook water, but that is just how Czech expresses it

This is perfectly normal Czech.

English separates cook and boil more clearly, but Czech uses vařit in a wider way.

Because it is the direct object of vařit, so it goes into the accusative case.

• nominative: voda = water
• accusative: vodu

So:

• vařit vodu = to boil water

This is another basic Czech pattern:

• the thing directly affected by the verb is often in the accusative

For example:

• mám čaj = I have tea
• piju kávu = I drink coffee
• vařím vodu = I boil water

Here na čaj means for tea.

It expresses purpose: the water is being boiled for making tea.

The preposition na is very often used in Czech to express purpose or intended use:

• peníze na jídlo = money for food
• čas na odpočinek = time for rest
• voda na čaj = water for tea

So vodu na čaj means water intended for tea.

Because in this sentence na takes the accusative case, and čaj has the same form in nominative and accusative singular.

So although the case does change grammatically, the form happens to stay the same:

• nominative: čaj
• accusative: čaj

That is very common with many masculine inanimate nouns in Czech.

Yes. Czech word order is flexible.

This sentence could also be said as:

• Budu vařit vodu na čaj, až přijdu do kuchyně.

That still means essentially the same thing.

The version

• Až přijdu do kuchyně, budu vařit vodu na čaj.

is very natural because it presents the time setting first, then the main action.

Czech word order often depends on emphasis, flow, and what information is already known.

Because Až přijdu do kuchyně is a subordinate clause, and Czech separates subordinate clauses with commas.

So the structure is:

• Až přijdu do kuchyně = subordinate time clause
• budu vařit vodu na čaj = main clause

Czech punctuation is quite strict about this, more so than English in some similar cases.

In natural English, it is usually closer to:

• I get to the kitchen
• I come into the kitchen
• I arrive in the kitchen

The verb přijít often implies reaching a place, not just the general act of coming.

So in this sentence, Až přijdu do kuchyně suggests the moment when the speaker gets there, and then starts boiling the water.