Learn Hardcore Portuguese (Portugal): Se o aparelho falhar, o aluguer inclui substituição. - If the device fails, the rental includes a replacement.

It’s the future subjunctive. Portuguese uses the future subjunctive after conjunctions like se (if), quando (when), logo que/assim que (as soon as) to talk about future or hypothetical situations.

Formation (from the pretérito perfeito 3rd person plural: falharam): remove -am → falhar-, then add endings.
Forms: se eu falhar, se tu falhares, se ele/ela falhar, se nós falharmos, se vós falhardes, se eles falharem.

Could I say Se o aparelho falhe or Se o aparelho falha?

Se o aparelho falhe: No, not for this meaning. With se introducing a future condition, Portuguese wants the future subjunctive: se o aparelho falhar.
Se o aparelho falha: This present indicative works for general/habitual truths (e.g., “If the device fails often, …”), but not for a specific future contingency. For that, stick to se … falhar.
If you want to use the present subjunctive, switch the conjunction: Caso o aparelho falhe, …

Why is there a comma after the first clause?

When a subordinate clause comes first in Portuguese, it’s standard to separate it with a comma: Se o aparelho falhar, … If you put the main clause first, you usually drop the comma: O aluguer inclui substituição se o aparelho falhar.

Can I reverse the order of the clauses?

Yes. O aluguer inclui substituição se o aparelho falhar is equally correct and common.

Why o aluguer and not o aluguel?

Because this is European Portuguese. Aluguer (masculine) is the European form; Brazilian Portuguese uses aluguel (also masculine). The verb in both varieties is alugar (“to rent/hire”).

What’s the difference between aluguer, arrendamento, and renda?

aluguer: hiring/rental of goods or services (cars, equipment, devices, bikes, etc.).
arrendamento: leasing/renting of real estate (flats, land); legal/contractual nuance.
renda: the rent (payment) for property; also means “lace” in other contexts. Not used for hiring devices.

Why is there no article before substituição in o aluguer inclui substituição?

Portuguese often omits the article before abstract, service-like nouns after verbs like incluir/oferecer when speaking generally: inclui entrega, inclui manutenção, inclui substituição. If you want to be specific, add the article or a complement: inclui a substituição do aparelho.

Could/should I say substituição do aparelho?

Yes, if you want to specify what is being replaced: Se o aparelho falhar, o aluguer inclui a substituição do aparelho. Without the complement, substituição is understood from context.

What exactly does aparelho mean here? Are there synonyms?

Aparelho means a device/appliance/piece of equipment (generic). Depending on context, you could use:

equipamento (equipment; often heavier-duty or professional)
dispositivo (device; more technical/IT)
máquina (machine; mechanical emphasis)

Why isn’t falhar reflexive (no se)?

Because falhar (“to fail”) isn’t pronominal in this sense. Some other verbs about breakdowns often are:

avariar-se (“to break down”): Se o aparelho se avariar, …
estragar-se (“to get damaged/spoil”): Se o aparelho se estragar, … These are good alternatives; falhar emphasizes failing to function/perform rather than a mechanical breakdown per se.

Is Se o aparelho avariar (without se) acceptable?

In European Portuguese, you’ll frequently hear the pronominal form (avariar-se). The non-pronominal avariar is also found and accepted, but avariar-se is more idiomatic when the device itself breaks down: Se o aparelho se avariar, …

Is inclui the right form? Could I say está incluída instead?

inclui is 3rd person singular present of incluir, agreeing with o aluguer: O aluguer inclui substituição.
You can rephrase to focus on the included item: A substituição está incluída no aluguer. Both are correct; the subject changes, so the agreement changes too.

Why no preposition after inclui?

Incluir is a transitive verb and takes a direct object: inclui substituição. If you switch to a passive/result construction, you’ll use a preposition: está incluída no aluguer (included in the rental).

How would I say “in case of failure” more nominally?

You can use a prepositional phrase:

Em caso de falha (do aparelho), o aluguer inclui substituição.
No caso de o aparelho falhar, o aluguer inclui substituição. This is a bit more formal/contract-like.

Any pronunciation tips (Portugal)?

aparelho: a-pa-RE-lyoo (the lh is like the “lli” in “million”)
aluguer: a-lu-GUER (hard g; the final r is a light tap in much of Portugal)
inclui: in-CLUI (the ui is one syllable, like “oo-ee”)
substituição: soob-stee-too-ee-SÃW (final -ção is nasal; the ç sounds like “s”)

Are there other triggers for the future subjunctive I should know?

Yes, besides se, you commonly use it after:

quando (when): Quando o aparelho falhar, …
logo que/assim que (as soon as): Assim que o aparelho falhar, …
onde/quiser/quando for-type clauses: quando for necessário, onde for aplicável, etc.