Learn Hardcore Portuguese (Portugal): Se o despertador falhar, faço uma sesta mais curta depois do almoço. - If the alarm clock fails, I take a shorter nap after lunch.

Because Portuguese uses the future subjunctive after se to talk about a real, possible future condition. Here, falhar is the 3rd person singular of the future subjunctive of falhar (it happens to look like the infinitive).

Present subjunctive: eu falhe, tu falhes, ele/ela falhe
Future subjunctive: eu falhar, tu falhares, ele/ela falhar
Present indicative: ele/ela falha

So Se o despertador falhar… = If the alarm fails (at some future point)… Falhe would be used after triggers like É possível que…, or to express wish/uncertainty not tied to a specific future condition. Falha would state a plain present fact: If the alarm fails (as it usually does)… which is not the intended meaning here.

Could I say farei or vou fazer instead of faço?

Yes, but the nuance changes:

faço = present indicative with a habitual or rule-like meaning: “When/If that happens, I (normally) do X.”
farei = simple future, more like a specific future outcome: “If it fails (that time), I will make/take a shorter nap.”
vou fazer = periphrastic future, often more immediate or planned: “If it fails, I’m going to take a shorter nap.”

All three are grammatical; choose based on whether you mean a general habit (faço) or a particular future occasion (farei/vou fazer).

How would I make the “more hypothetical/unlikely” version?

Use the imperfect subjunctive in the if-clause and the conditional in the main clause:

Se o despertador falhasse, faria uma sesta mais curta depois do almoço. Meaning: “If the alarm were to fail, I would take a shorter nap after lunch.”

Can I use quando instead of se?

quando = when/whenever, implying you expect it to happen at some point: Quando o despertador falhar, faço…
se = if, expressing a possibility: Se o despertador falhar, faço…

Use quando only if you’re treating the alarm’s failure as expected or inevitable.

Is fazer uma sesta the natural way to say “take a nap” in Portugal?

Yes. Common collocations in European Portuguese include:

fazer uma/a sesta
dormir a sesta
tirar uma sesta (heard, but less idiomatic than the two above)

In Brazil, you’d more often hear tirar um cochilo or tirar uma soneca.

Why is it curta (feminine) and not curto?

Because sesta is feminine (a sesta), so the adjective agrees in gender and number: uma sesta mais curta. With a masculine noun you’d say curto: um cochilo mais curto.

Is mais curta a complete comparative even without do que?

Yes. When the comparison target is understood from context (e.g., shorter than my usual nap), Portuguese can simply say mais curta. If you explicitly compare, use do que: uma sesta mais curta do que a habitual.

Is the comma after the se-clause required?

Yes, when the subordinate clause comes first, you normally put a comma: Se o despertador falhar, faço… If the main clause comes first, you usually don’t need a comma: Faço uma sesta mais curta depois do almoço se o despertador falhar.

What does do in depois do almoço stand for?

Do is the contraction of de + o. So:

depois de o almoço → depois do almoço This is standard with articles. You might also see:
depois de almoçar (after having lunch; uses a verb)
após o almoço (more formal)
a seguir ao almoço (European Portuguese, very natural)

Can I omit the article and say depois de almoço?

It’s heard in European Portuguese, especially in set time expressions, but depois do almoço is more neutral and specific to that day’s lunch. Depois de almoçar is also very common and avoids the article question entirely.

Is falhar the best verb for an alarm that doesn’t go off?

Falhar is fine and natural in Portugal. Alternatives:

Se o despertador não tocar… (if the alarm doesn’t ring)
Se o alarme falhar… (if the alarm fails)
Se o despertador avariar… (if it breaks down; implies a defect, not just missing the time) In Brazil you may also hear pifar (slangy) for devices failing.

Could I say Se falhar o despertador instead of Se o despertador falhar?

Yes. Verb–subject inversion in subordinate clauses is possible in European Portuguese: Se falhar o despertador, faço… Both orders are fine; the original order (subject before verb) is the most straightforward for learners.

Why uma sesta and not a sesta?

Both work, but they feel different:

fazer a sesta often refers to “the (habitual) nap,” a routine daily siesta.
fazer uma sesta presents it as “a nap” on a given occasion (or a type/amount), which matches the idea of adjusting its length.

Is pequena okay instead of curta?

For time/duration, curta is the idiomatic choice. Pequena mainly refers to physical size or general smallness. With naps, sesta/sono curto means short in duration.

Pronunciation tips for tricky bits?

despertador: stress on the last syllable dor; the single r between vowels is a tapped r in most of Portugal.
falhar: lh is a palatal sound like the “lli” in million; final r is a strong uvular/“guttural” sound in much of Portugal.
faço: ç is an s sound; the a is open (FA-so).
sesta: initial s is unvoiced (like English s).
depois: the oi is like English “oy.”
almoço: ç is s; stress falls on the second syllable (al-MO-so).

Is despertador only a physical alarm clock?

Despertador is the standard word for an alarm clock, but it’s also understood for a phone alarm in Portugal (o despertador do telemóvel). Alarme is also used for the alarm function itself (o alarme não tocou).