Learn Hardcore Portuguese (Portugal): Se o aluguer terminar por volta de sexta‑feira, definimos outra data. - If the rental ends around Friday, we’ll set another date.

It’s the future subjunctive. In Portuguese, when you talk about a possible future condition with se (if), you normally use the future subjunctive in the if‑clause:

terminar: eu/ele terminar, tu terminares, nós terminarmos, eles terminarem. So, Se o aluguer terminar… = If the rental ends (at some future point)…

Can I say se o aluguer termina or se o aluguer terminará?

se o aluguer termina is grammatical but usually describes habitual or general conditions, not a specific future possibility.
se o aluguer terminará is wrong: after se you don’t use the future indicative. Use the future subjunctive: se o aluguer terminar.

Why is definimos in the present if it refers to the future?

Portuguese often uses the present indicative to talk about the future in the main clause of an if‑sentence: Se chover, ficamos em casa. Likewise, Se … terminar, definimos… means we’ll set another date. You could also say:

definiremos (future; a bit more formal)
vamos definir (periphrastic future; very common)

Is definimos here present or past?

Formally it could be either, because nós definimos is the same in the present and the simple past. Context disambiguates it. After Se o aluguer terminar…, it’s understood as present with future meaning. To avoid any ambiguity, use vamos definir or definiremos.

What’s the difference between aluguer, arrendamento, and renda? And how about aluguel?

aluguer (EU PT): renting/hire in general (cars, equipment, short‑term).
arrendamento: leasing/renting of property (flats, offices).
renda: the rent payment (monthly amount), also “income”.
aluguel is the Brazilian Portuguese word for EU PT aluguer. In Portugal, prefer aluguer.

Does por volta de work with days of the week, or only with clock times?

It works with both. por volta de sexta‑feira = around Friday; por volta das cinco = around five o’clock. Other options: lá para sexta, mais ou menos na sexta‑feira, cerca de (more numeric).

Do I need an article after por volta de here?

Not with a bare weekday: por volta de sexta‑feira (no article). Use an article when the noun phrase normally takes one:

por volta do dia 15 (the 15th takes the article)
por volta de maio (months are usually bare) Avoid por volta da sexta‑feira in this sense.

Is por volta de the same as cerca de or aproximadamente?

Broadly yes (“about/around”), but:

por volta de is the most idiomatic for time expressions.
cerca de leans numeric/quantitative (cerca de 20 minutos), though it can work with time.
aproximadamente is more formal.

Could I use acabar instead of terminar?

Yes: Se o aluguer acabar por volta de sexta‑feira… is fine. acabar and terminar often overlap. Just avoid acabar por + infinitive here, because acabar por means “to end up (doing something)”, which is a different structure.

Is the comma after the if‑clause required?

Yes, when the se‑clause comes first: Se…, [comma] …. If you invert the order, you normally drop the comma: Definimos outra data se o aluguer terminar….

Is sexta‑feira always hyphenated and capitalized?

Spelling: sexta‑feira with a hyphen; in informal contexts you can shorten to sexta.
Capitalization: lowercase in Portuguese.
With a specific day you often use an article and preposition: na sexta‑feira (on Friday).

Could por volta de confuse people with “terminar por” (to end up doing something)?

No, because por volta de is a fixed expression meaning “around/about (time)”. In your sentence the structure clearly parses as terminar + por volta de [time], not terminar por [verb].

Can I use caso instead of se?

Yes, but the grammar shifts: with caso you use the present subjunctive, not the future subjunctive.

Caso o aluguer termine por volta de sexta‑feira, definimos outra data. It’s a bit more formal than se.

How would I say this as a less likely or purely hypothetical condition?

Use the “unreal” patterns:

Present hypothetical: Se o aluguer terminasse por volta de sexta‑feira, definiríamos outra data.
Past counterfactual: Se o aluguer tivesse terminado por volta de sexta‑feira, teríamos definido outra data.

Are there better verbs than definir for scheduling?

All are fine but with nuances:

marcar: to set/schedule (very common) — marcamos outra data.
agendar: to schedule (slightly formal).
definir: to decide/fix; focuses on deciding rather than calendar booking.
combinar: to agree on (informal, collaborative).