Learn Hardcore Portuguese (Brazil): Se eu não pagar o aluguel hoje, vou ficar nervoso. - If I don’t pay the rent today, I’ll get nervous.

Because this is a future/uncertain condition (“if I don’t pay (today)…”) and Brazilian Portuguese typically uses se + futuro do subjuntivo in that situation.

pagar here is the future subjunctive form (which looks the same as the infinitive for regular -ar verbs).
Using Se eu não pago sounds like a habitual/general statement (“If I don’t pay (in general)…”), and it’s much less natural for a specific future consequence.

Is pagar here really subjunctive even though it looks like the infinitive?

Yes. For many regular verbs, the future subjunctive form is identical to the infinitive in some persons.
Conjugation of pagar (futuro do subjuntivo):

se eu pagar
se você/ele pagar
se nós pagarmos
se vocês/eles pagarem

So Se eu não pagar is “If I don’t pay…”

Could I say Se eu não pagar o aluguel hoje, ficarei nervoso instead of vou ficar nervoso?

Yes. Ficarei nervoso (simple future) is correct and means the same thing.
In everyday Brazilian Portuguese, ir + infinitivo (vou ficar) is extremely common and often sounds more conversational. Ficarei can sound a bit more formal or “written.”

What does ficar mean here—“stay” or “become”?

Here ficar + adjective means to become / to end up in a state.
So vou ficar nervoso = “I’m going to get nervous.”
(“To stay” can also be ficar, but context usually makes it clear.)

Why is there a comma after hoje?

It’s separating the conditional clause from the result clause:

Se eu não pagar o aluguel hoje, (condition)
vou ficar nervoso. (result)
The comma is very common and recommended in writing, especially when the se-clause comes first.

Can I omit eu and just say Se não pagar o aluguel hoje, vou ficar nervoso?

Yes, that’s natural in Portuguese because the verb form already signals the subject.
Including eu adds emphasis/contrast (like “If I don’t pay…”).

Why is it o aluguel and not just aluguel?

Both can appear, but pagar o aluguel is very common because the rent is treated as a specific, known payment (“the rent”).
You might also hear pagar aluguel in a more general sense (like “pay rent (as a concept)”).

Does aluguel mean the apartment/lease, or the payment?

In pagar o aluguel, aluguel means the rent payment.
(“Rent” as a concept can be either the arrangement or the payment, but with pagar it’s clearly the payment.)

Where does não go, and can it move?

In simple clauses, não usually comes right before the verb: não pagar.
So Se eu não pagar... is the normal placement. You generally wouldn’t put não after the verb in modern Brazilian Portuguese.

Why is nervoso masculine—what if the speaker is a woman?

Adjectives agree with the person being described.

Male speaker: vou ficar nervoso
Female speaker: vou ficar nervosa
Plural would be nervosos / nervosas depending on the group.

Is hoje required, and where else could it go?

It’s not required, but it adds urgency/specific timing.
Common placements:

Se eu não pagar o aluguel hoje, vou ficar nervoso. (most natural)
Se eu não pagar hoje o aluguel, ... (possible, but a bit less smooth)
Se hoje eu não pagar o aluguel, ... (possible for emphasis on “today”)

Could I use caso instead of se?

Yes, caso can mean “in case / if,” and it often sounds a bit more formal:

Caso eu não pague o aluguel hoje, vou ficar nervoso.
Notice that with caso, Portuguese commonly uses the present subjunctive (pague), not the future subjunctive.

How would this sound in more informal speech?

A very natural informal version might be:

Se eu não pagar o aluguel hoje, eu vou ficar nervoso. (extra eu is common in speech)
Or even:
Se eu não pagar o aluguel hoje, vou ficar bem nervoso. (“really nervous”)