Learn Hardcore Portuguese (Portugal): Quanto mais estudares, mais depressa te organizas para devolver o que te emprestam. - The more you study, the faster you get yourself organised to return what they lend you.

It’s the Portuguese way to say “the more X, the more Y” (or “the more X, the less Y”). Each half is introduced by quanto mais/menos and then mais/menos. You normally separate the halves with a comma: Quanto mais X, mais Y. It works with verbs, adjectives, adverbs, or nouns (e.g., Quanto mais livros leres, melhores notas terás).

Why is estudares used here? What tense/mood is it?

Estudares is the future subjunctive (futuro do conjuntivo) of estudar. In EP, this mood is common after correlative patterns like quanto mais, when the action is viewed as prospective or conditional. Forms: eu/ele estudar, tu estudares, nós estudarmos, vós estudardes, eles estudarem.

Could I use the present instead: Quanto mais estudas, mais depressa te organizas? Or use the future in the second clause?

Yes to both, with small nuance differences:

Present + present (general/habitual): Quanto mais estudas, mais depressa te organizas.
Future subjunctive + future indicative (more clearly future-looking): Quanto mais estudares, mais depressa organizar-te-ás. All are acceptable; pick the one that best matches your intended time frame.

Why te organizas and not organizas-te?

Both are correct in European Portuguese. Starting a clause with an initial adverbial like mais depressa commonly draws the clitic before the verb (proclisis): mais depressa te organizas. Enclisis is also fine and a bit more formal: mais depressa organizas-te. If you use the future, enclisis with a hyphen is mandatory: organizar-te-ás.

What does depressa mean? Can I say rápido/rapidamente instead?

Depressa means “quickly/fast/soon (in the sense of speed).” Mais depressa = “faster/sooner.” In Portugal, depressa is very common and idiomatic. Alternatives:

rapidamente (more formal/neutral adverb),
rápido (very common in speech as an adverb, though prescriptively an adjective),
Note: mais cedo means “earlier/sooner (in time),” not necessarily “faster.”

What exactly does o que mean here? Why not just que?

O que is a nominal relative meaning “what/that which.” Without a head noun, Portuguese needs o que, not bare que. So devolver o que te emprestam = “to return what they lend you.” You could also say aquilo que for emphasis: devolver aquilo que te emprestam.

Why is it o que te emprestam and not o que emprestam-te?

Because que (a relative pronoun) triggers proclisis in EP: the clitic must come before the verb. So it’s o que te emprestam, not o que emprestam-te. Other proclisis triggers include negatives, interrogatives, certain adverbs, and subordinating conjunctions.

Who is the subject of emprestam?

It’s an indefinite “they/people.” That’s a common way in Portuguese to avoid a passive. Alternatives:

Future-oriented relative: o que te emprestarem (“whatever they lend you [in the future]”).
Passive: o que te é emprestado (present) or o que te for emprestado (future/contingent).

Why para devolver and not a devolver or por devolver?

Para + infinitive expresses purpose (“in order to”). A + infinitive often marks progressive/onset or follows certain verbs (e.g., estar a devolver = “to be returning”). Por + infinitive often means “still to [be done]/pending” (e.g., livros por devolver = “books still to return”). So here, para is the right choice.

Is the comma after Quanto mais estudares necessary?

Yes, you normally separate the two halves: Quanto mais …, mais …. It’s standard punctuation and helps readability.

How would this change with você or plural addressees?

With você (3rd person forms): Quanto mais estudar, mais depressa se organiza para devolver o que lhe emprestam.
With vocês (3rd person plural): Quanto mais estudarem, mais depressa se organizam para devolver o que vos emprestam / o que emprestam a vocês. Notes: In EP, reflexive with você(s) is se. Indirect object can be lhe/lhes, vos (more formal/old-fashioned), or a prepositional phrase (a você(s)).

Could estudares be the personal infinitive here?

The form looks the same as the personal infinitive (tu estudares), but in this construction it’s the future subjunctive. A quick check: the future subjunctive fits after things like quando/se/quanto (e.g., Quando estudares…), whereas the personal infinitive typically follows a preposition (para/sem/por estudares). Here, quanto mais calls for the future subjunctive.