Se eu ficar cansado, vou descansar.

If I get tired, I will rest.

Breakdown of Se eu ficar cansado, vou descansar.

• eu: I
• cansado: tired
• ir: to go (future auxiliary)
• se: if
• descansar: to rest
• ficar: to become / to get

Questions & Answers about Se eu ficar cansado, vou descansar.

Why is it se eu ficar and not se eu fico?

Because Portuguese typically uses the future subjunctive after se when you’re talking about a possible future condition (something that may or may not happen): Se eu ficar cansado... = If I get tired (at some point in the future)...
Se eu fico cansado sounds more like a habit/general truth (roughly If/when I get tired (in general), I rest), and even then many speakers prefer quando eu fico cansado or sempre que eu fico cansado for the habitual meaning.

What tense/mood is ficar in here?

It’s the future subjunctive:

• (se/quando) eu ficar
• você/ele ficar
• nós ficarmos
• vocês/eles ficarem
  Portuguese uses this form a lot in clauses introduced by se, quando, assim que, logo que, etc., when referring to the future.

Does ficar cansado mean to be tired or to get tired?

In this structure, ficar + adjective usually means to become / to get that state.
So ficar cansado is most naturally to get tired (or to become tired).
If you want simply to be tired as a state, you’d typically use estar: Se eu estiver cansado, vou descansar.

So what’s the difference between Se eu ficar cansado and Se eu estiver cansado?

• Se eu ficar cansado: focuses on the change (you become tired).
• Se eu estiver cansado: focuses on the state (you am tired).
  In everyday use, both can work depending on what you mean, but ficar often sounds like if I end up getting tired.

Why is vou descansar used instead of descansarei?

Vou descansar is the very common near future construction (ir + infinitive) and often sounds more natural in conversation.
Descansarei (simple future) is grammatical but can sound more formal, emphatic, or written: Se eu ficar cansado, descansarei.

Can I drop the pronoun eu?

Yes. Portuguese often omits subject pronouns because the verb form already signals the subject:

• Se eu ficar cansado, vou descansar.
• Se ficar cansado, vou descansar.
  Both are correct; keeping eu can add clarity or emphasis.

Why is there a comma in the middle?

When the if-clause (Se eu ficar cansado) comes first, Portuguese normally uses a comma before the main clause:

• Se eu ficar cansado, vou descansar.
  If you invert the order, the comma is usually not used:
• Vou descansar se eu ficar cansado.

What does vou literally mean here?

Vou is the present tense of ir (to go) for eu: eu vou = I go / I’m going.
In vou + infinitive, it functions like going to in English to express a future plan/decision: vou descansar = I’m going to rest / I’ll rest.

Why is it cansado and not cansada?

Because cansado/cansada agrees with the speaker’s gender (or the subject’s gender):

• If a man says it: Se eu ficar cansado...
• If a woman says it: Se eu ficar cansada...
  Same rule with plural: cansados / cansadas.

Can I replace se with quando here?

You can, but it changes the meaning:

• Se eu ficar cansado, vou descansar. = If I get tired (maybe I will, maybe I won’t), I’ll rest.
• Quando eu ficar cansado, vou descansar. = When I get tired (it’s expected/assumed to happen), I’ll rest.
  Both use the future subjunctive (ficar) when referring to the future.