Když má můj syn žízeň, pije džus z láhve.

Here, když means when, and in this kind of sentence it often feels like whenever in English.

So the idea is not just one single moment, but a repeated situation:

• Když má můj syn žízeň, pije džus z láhve.
• Whenever my son is thirsty, he drinks juice from a bottle.

Czech když can sometimes overlap with English if in everyday speech, but in this sentence the meaning is clearly temporal: when / whenever.

Because Když má můj syn žízeň is a subordinate clause, and Czech normally separates subordinate clauses from the main clause with a comma.

So the structure is:

• Když má můj syn žízeň = subordinate clause
• pije džus z láhve = main clause

That is why the comma is required:

• Když má můj syn žízeň, pije džus z láhve.

If you reverse the order, you still use a comma:

• Můj syn pije džus z láhve, když má žízeň.

Because Czech often expresses physical states with mít (to have) + a noun.

So:

• mít žízeň = literally to have thirst
• idiomatically = to be thirsty

This is very normal Czech. English uses to be here, but Czech uses to have.

Similar examples:

• mít hlad = to be hungry
• mít strach = to be afraid
• mít čas = to have time

So má žízeň is exactly the natural Czech way to say is thirsty.

Because můj syn is the subject of the sentence, so it is in the nominative case.

• můj = nominative singular masculine
• syn = nominative singular masculine animate

The possessive word můj has to agree with the noun it belongs to.

So:

• můj syn = my son
• moje syn is not correct standard Czech
• mého syna would be a different case, not the subject form

For example:

• Vidím mého syna. = I see my son.
  Here mého syna is accusative.

But in your sentence, my son is doing the action, so Czech uses the nominative:

• můj syn

Yes. This sentence most naturally describes a habitual or general situation.

In Czech, the present tense is often used for repeated actions or general truths:

• Když má můj syn žízeň, pije džus z láhve.

This means something like:

• When / whenever my son is thirsty, he drinks juice from a bottle.

So it does not have to mean that this is happening right now. It usually means this is what he generally does.

English does the same thing in many cases:

• When he is thirsty, he drinks juice.

Because pije comes from the imperfective verb pít, which is the normal choice for a repeated, habitual, or ongoing action.

• pít = to drink
• pije = he/she drinks, is drinking

In this sentence, the meaning is general and habitual, so pije fits naturally.

By contrast:

• vypít / vypije is perfective
• it suggests drinking something up, finishing it

So:

• pije džus = he drinks juice
• vypije džus = he drinks up the juice

In a habitual sentence like this, pije is the expected form.

Džus is the direct object of pije, so it is in the accusative case.

However, džus is a masculine inanimate noun, and in the singular, masculine inanimate nouns often have the same form in nominative and accusative.

So:

• nominative: džus
• accusative: džus

That is why it looks unchanged.

This is very common in Czech. For example:

• čaj → piju čaj
• chléb → jím chléb
• džus → pije džus

So the case is changing grammatically, but the form happens to stay the same.

Because the preposition z (from / out of) requires the genitive case.

The basic dictionary form is:

• láhev = bottle

After z, it changes to the genitive singular:

• z láhve = from the bottle / out of a bottle

So the pattern is:

• láhev → nominative
• z láhve → genitive after z

This is an important Czech rule: many prepositions require a specific case.

Compare:

• z láhve = from the bottle
• v láhvi = in the bottle
• do láhve = into the bottle

Each preposition changes the case.

It can mean either, depending on context.

Literally, z láhve means from/out of the bottle. In natural English, that may become:

• from the bottle
• out of a bottle
• sometimes simply from a bottle

Because Czech has no articles, the sentence itself does not force the or a here. Context decides.

So z láhve tells you the source or container, but not by itself whether the bottle is specific or nonspecific.

Czech word order is fairly flexible, but not completely free. The sentence you have is a very normal, neutral version.

Neutral order:

• Když má můj syn žízeň, pije džus z láhve.

You could also say:

• Můj syn pije džus z láhve, když má žízeň.

That still makes sense, but the focus feels a little different.

Because Czech uses case endings, word order is often used for emphasis, topic, and information structure, not just for basic grammar. So you can move things, but some orders sound more natural than others.

For a learner, the version you have is a good model.

Because Czech does not have articles like English the and a/an.

So Czech often leaves that information to context.

In this sentence:

• můj syn already clearly means my son
• džus could be juice, the juice, or some juice, depending on context
• z láhve could be from the bottle or from a bottle

English requires articles, but Czech usually does not.

That means when translating into English, you often have to choose the most natural article from context.

A rough guide:

• džus: the dž sounds like English j in jam
• u sounds like oo in food
• so džus is roughly joos

For žízeň:

• ž sounds like the s in measure
• í is a long ee
• the final ň is a soft n, a bit like Spanish ñ or ny

So a rough approximation is something like:

• žízeň ≈ zhee-zeñ

Not exact, but close enough for a learner.

Also:

• když has ž, the same sound as in žízeň
• láhve starts with a long á, so hold that vowel slightly longer

If you want, I can also break the whole sentence down word by word for pronunciation.