Learn Hardcore Portuguese (Portugal): Quanto mais água beberes, menos sede tens. - The more water you drink, the less thirsty you are.

It’s a correlative comparative: “the more …, the less ….” Use it to express a proportional relationship between two quantities or degrees. You can also invert it:

Quanto mais X, mais Y.
Quanto menos X, mais Y.
Quanto mais X, menos Y.
Quanto menos X, menos Y.

What form is beberes, and why is it used here?

Beberes is the future subjunctive, 2nd person singular (tu). After quanto (mais/menos) in this correlative pattern, Portuguese typically uses the future subjunctive in the first clause to express an open or general condition (similar to English “the more you drink…”).

How can I tell this isn’t the personal infinitive?

For many regular verbs the future subjunctive and the personal infinitive look the same (e.g., beberes). You tell them apart by context:

After quanto/quando/se/assim que, it’s future subjunctive.
After prepositions like para/por/sem/ao, it’s personal infinitive. A clear contrast with an irregular verb: Quanto mais sede tiveres… (future subj.) vs para teres sede (personal infinitive).

Could I use the present instead of the future subjunctive in the first clause? For example, Quanto mais água bebes…?

Yes, in everyday European Portuguese you will hear the present indicative: Quanto mais água bebes, menos sede tens. The future subjunctive is a bit more neutral/standard and often preferred in writing. The meaning is essentially the same: a general proportional relation.

Why is it tens (present indicative) in the second clause, not tenhas (subjunctive) or terás (future)?

Because the second clause states the resulting fact in general terms. Present indicative is the natural choice for general truths: menos sede tens. You can use the future if you’re talking about a future time frame, e.g., Quanto mais água beberes hoje, menos sede terás amanhã. The subjunctive (tenhas) would be wrong here.

How would the sentence change with other subjects like você, ele/ela, nós, vocês?

Você/ele/ela: Quanto mais água beber, menos sede tem (ou terá).
Nós: Quanto mais água bebermos, menos sede temos (ou teremos).
Vocês/eles/elas: Quanto mais água beberem, menos sede têm (ou terão). Note the future subjunctive forms: eu beber, tu beberes, ele/você beber, nós bebermos, vós beberdes (rare), eles/vocês beberem.

Why is there no article before água?

Because água is a mass noun used generically/indefinitely here. With mais/menos + mass nouns, Portuguese commonly omits the article: mais água, menos açúcar. Use the article when the reference is specific: Quanto mais da água do cantil beberes…

Is menos variable? Do I ever say something like “menas”?

Menos is invariable. It never changes form. “Menas” is incorrect.

Why is it menos sede tens instead of the more English-like order tens menos sede?

Both are correct. Tens menos sede is the neutral order. Menos sede tens front-loads the focus on the result (“less thirst”) and is quite natural in this correlative pattern. It’s a stylistic choice.

Can I say Quanto mais beberes água instead of Quanto mais água beberes?

Portuguese strongly prefers placing mais before the noun it quantifies: mais água beberes sounds more natural. Quanto mais beberes água is possible but less idiomatic here. If mais modifies the verb/adverb, it can go elsewhere (e.g., Quanto mais depressa beberes…).

Can I include tanto in this structure, like Quanto mais…, tanto menos…?

Yes. Tanto is optional and adds emphasis or balance: Quanto mais água beberes, tanto menos sede tens. In everyday speech it’s often omitted.

Could I flip it to express the opposite relation?

Absolutely: Quanto menos água beberes, mais sede tens.

Is there a difference between this and using se (“if”): Se beberes mais água, tens menos sede?

They can express the same idea, but the nuance differs:

Quanto mais…, menos… highlights a proportional, gradable relationship.
Se… states a condition without explicitly stressing proportionality.

How would this look in Brazilian Portuguese?

Commonly with você and third-person forms: Quanto mais água você beber, menos sede você tem/terá. BP also often uses estar com sede: Quanto mais água você beber, menos com sede você fica.

Why ter sede and not estar com sede?

In European Portuguese, ter sede (“to have thirst”) is the default. Estar com sede exists and is understood, but ter sede is more common in EP. In BP, estar com sede is very frequent, though ter sede is also correct.

Any pronunciation tips for European Portuguese here?

Quanto ≈ “KWAN-tu” (first vowel nasal).
mais ends with a “sh” sound: “maish.”
água stress on á: “AH-gwa.”
beberes ≈ “bɨ-BE-rɨsh”: unstressed e reduces; single r is a light tap; final s is “sh.”
menos ≈ “ME-nush” (final s “sh”).
tens has a nasal vowel and ends with “sh”: roughly “tensh.”

Do I need the comma?

Yes. Separate the two correlated clauses with a comma: Quanto mais…, menos…. You can invert the order, but still keep a comma: Menos sede tens, quanto mais água beberes.