Když mám žízeň, piji vodu ze sklenice.

Když means when in this sentence.

It introduces a subordinate clause: Když mám žízeň = When I am thirsty.

A very common pattern in Czech is:

Když + clause, main clause

So here:

• Když mám žízeň = when I’m thirsty
• piji vodu ze sklenice = I drink water from a glass

Also notice the comma after the když-clause. Czech normally uses a comma before the main clause in this structure.

Czech often expresses this idea with the idiom mít žízeň, literally to have thirst.

So:

• mám žízeň = I’m thirsty
• literally: I have thirst

This is extremely common and natural in Czech.

You can also say jsem žíznivý / žíznivá (I am thirsty), but that sounds more adjectival and is often less everyday than mám žízeň.

So for a learner, it is best to remember:

• mít hlad = to be hungry
• mít žízeň = to be thirsty

In Czech, subject pronouns are often omitted because the verb ending already shows the person.

Here:

• mám = I have
• piji = I drink

Because the verb already tells you it is I, Czech usually does not need já.

So:

• Když mám žízeň, piji vodu ze sklenice. = natural
• Když já mám žízeň, já piji vodu ze sklenice. = grammatically possible, but usually unnecessary and would sound emphatic

You would use já only if you want contrast or emphasis.

Yes. Both piji and piju are used.

For the verb pít (to drink):

• piji = more standard/written/formal
• piju = very common in everyday speech

So both of these are natural:

• Když mám žízeň, piji vodu ze sklenice.
• Když mám žízeň, piju vodu ze sklenice.

As a learner, it is good to recognize both. If you want a safer textbook-style form, use piji.

Because vodu is the accusative singular form of voda.

The verb pít takes a direct object, and direct objects are usually in the accusative case.

So:

• nominative: voda = water
• accusative: vodu = water (as the thing being drunk)

Compare:

• Voda je studená. = The water is cold.
  
  • voda is the subject, so nominative
• Piji vodu. = I drink water.
  
  • vodu is the object, so accusative

The preposition is basically z (from, out of), but it often changes to ze for easier pronunciation.

So:

• z = basic form
• ze = pronunciation-friendly variant used before some consonant combinations

Since sklenice begins with skl-, saying z sklenice would be awkward, so Czech uses:

• ze sklenice

This is similar to how some languages add a vowel to make pronunciation smoother.

Common examples:

• z domu = from the house
• ze školy = from school
• ze sklenice = from a glass

After z / ze, Czech uses the genitive case.

So ze sklenice means from a glass, and sklenice here is genitive singular.

The noun is:

• nominative singular: sklenice
• genitive singular: sklenice

In this particular noun, the nominative and genitive singular happen to look the same, which can be confusing for learners. But grammatically, after ze, it is genitive.

It can mean from a glass or out of a glass, depending on context.

Czech does not have articles like a and the, so sklenice by itself does not tell you whether it is:

• a glass
• the glass

You figure that out from context.

So piji vodu ze sklenice could be understood as:

• I drink water from a glass
• I drink the water out of the glass
• I’m drinking from the glass

depending on the situation.

Yes, it could be reversed.

This sentence begins with the time/condition clause:

• Když mám žízeň, piji vodu ze sklenice.
  = When I’m thirsty, I drink water from a glass.

But Czech can also say:

• Piji vodu ze sklenice, když mám žízeň.

Both are grammatical. The version with Když... first sounds very natural because it sets up the situation first.

Czech word order is more flexible than English, but it is not random. It often changes emphasis, topic, or style.

Most naturally, it expresses a habitual / general meaning:

• When I’m thirsty, I drink water from a glass.

The verb piji is imperfective, which fits repeated, usual, or ongoing actions.

So the sentence sounds like a general statement about what someone does in that situation, not a one-time completed event.

If you wanted a more specific one-time action in context, Czech would often use a different structure or rely on context.

A rough learner-friendly guide is:

žízeň ≈ zhee-zeny (but very short at the end)

Important points:

• ž sounds like the s in measure
• í is a long ee
• the final ň is a soft n, similar to the ny sound in canyon (though not exactly the same)

So try something like:

• ží → zhee
• zeň → zeny with a very light final y-like glide

Also, the stress in Czech is almost always on the first syllable:

• ŽÍ-zeň

In this sentence, sklenice most naturally means a drinking glass.

That is the normal everyday meaning of the word.

So:

• sklenice vody = a glass of water
• pít ze sklenice = to drink from a glass

In other contexts, it is still a glass container of some kind, but here the meaning is clearly the ordinary drinking vessel.

Native speakers would strongly prefer ze sklenice.

Using z sklenice is not the normal standard form because the cluster z sk- is awkward to pronounce. Czech solves this by using ze.

So for correct, natural Czech, use:

• ze sklenice

not:

• z sklenice

As a learner, it is worth memorizing z / ze as a pair, just like v / ve.