Když máš žízeň, měl bys pít vodu, ne jen džus.

Here když most naturally means when, but in a sentence about general advice or a repeated situation, English might also use if.

So:

• Když máš žízeň... = When you're thirsty...
• In natural English, If you're thirsty... can also fit the same idea.

In Czech, když often covers both kinds of meaning depending on context.

Czech uses the expression mít žízeň, literally to have thirst, to mean to be thirsty.

So:

• mám žízeň = I am thirsty
• máš žízeň = you are thirsty

This is very similar to another common Czech expression:

• mít hlad = to be hungry

You can also say jsi žíznivý / žíznivá for you are thirsty, but máš žízeň is the more common everyday phrasing.

Grammatically, žízeň is the object of máš, so it is in the accusative case.
But the noun žízeň has the same form in the nominative and accusative singular.

So even though the case changes, the word looks the same:

• nominative: žízeň
• accusative: žízeň

This is normal for some Czech nouns.

Měl bys means you should.

It is formed from:

• měl = past form of mít for masculine singular
• bys = conditional particle used with you

Together with an infinitive, it gives advice or recommendation:

• měl bys pít = you should drink

It is softer than a strong command like:

• musíš pít = you must drink

So měl bys is often the natural choice for giving advice.

Because Czech expresses should here with the structure:

past-form adjective/participle + conditional particle + infinitive

So:

• měl bys pít = you should drink
• měla bys pít = you should drink, said to a woman
• měl bych pít = I should drink

You need all the parts:

• měl / měla carries gender and number
• bych / bys / by / bychom / byste / by marks the conditional
• the infinitive gives the action: pít

In standard Czech, bych, bys, by, bychom, byste are clitics. These are short unstressed words that usually go in an early position in the clause, and in this construction they normally follow the participle-like word:

• měl bys
• měla bys
• měl bych

So měl bys pít is the normal standard order.

You may sometimes hear different word orders in speech for emphasis, but for learners, měl bys is the form to remember.

Yes. Měl is the masculine singular form, so this version is addressed to one male person.

If you were speaking to a woman, you would say:

• Když máš žízeň, měla bys pít vodu, ne jen džus.

Other versions:

• formal singular: Když máte žízeň, měl byste / měla byste...
• informal plural: Když máte žízeň, měli byste...
• all-female plural: měly byste...

So the part měl / měla / měli / měly changes for gender and number.

Because pít is imperfective, and that fits a general recommendation or habitual action:

• měl bys pít vodu = you should drink water

This means something like drinking water is what you should do in that situation.

If you used vypít, that would emphasize drinking something up / finishing it, often a specific amount:

• Měl bys vypít sklenici vody. = You should drink a glass of water.

So:

• pít = general action, ongoing/habitual
• vypít = finish drinking a specific amount

Both are direct objects of pít, so both are in the accusative case.

But they decline differently:

• voda → vodu in the accusative singular
• džus stays džus in the accusative singular

That is because voda is a feminine noun with a visible accusative ending, while džus is an inanimate masculine noun whose accusative singular is the same as the nominative.

So:

• pít vodu
• pít džus

It means not just juice or not only juice.

So the full idea is:

• drink water, not just juice

The speaker is not saying don't drink juice at all.
They are saying that juice alone is not enough / water is the better thing to drink when you're thirsty.

In this sentence, ne jen džus clearly means not just juice.

You will also see nejen written as one word, especially in patterns like:

• nejen ..., ale i ... = not only ..., but also ...

For a learner, the safest takeaway is:

• ne jen džus = not just juice
• nejen ... ale i ... = not only ... but also ...

In real usage, there can be some overlap, but here the separated form makes the meaning very clear.

Because Když máš žízeň is a subordinate clause introduced by když.

Czech normally separates subordinate clauses with a comma:

• Když máš žízeň, měl bys pít vodu.

This is similar to English:

• When you're thirsty, you should drink water.

So the comma is required in standard Czech spelling.

A few pronunciation points:

• když: the ž sounds like the s in measure
• žízeň: starts with a long ží-; the final -eň is softer than it may look to an English speaker
• měl: ě often softens the consonant before it
• džus: dž sounds like the j in jam

A rough learner-friendly approximation might be:

• Když ≈ gdyzh
• žízeň ≈ zhee-zeny (very approximate)
• džus ≈ joos

But it is best to learn these from audio, because Czech soft consonants and vowel length are hard to capture well with English spelling.