Se eu não pagar o aluguel hoje, vou ficar nervoso.

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.

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…”

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.”

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.)

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.

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…”).

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)”).

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.)

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.

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.

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”)

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.

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”)