Se eu ficar com dor nas costas, vou descansar em casa.

Because after se when you are talking about a possible future situation, Portuguese normally uses the future subjunctive.

• se eu ficar = if I get / if I end up / if I become
• se eu fico would sound wrong in this kind of sentence

This is a very common pattern in Portuguese:

• Se eu tiver tempo, vou ajudar.
• Se ele chegar cedo, sairemos.

So in your sentence, se eu ficar com dor nas costas means if I get back pain / if my back starts hurting.

Here ficar com means to come down with, to get, or to end up having something.

So:

• ficar com dor nas costas = to get back pain
• literally, it is something like to end up with pain in the back

This is very common in Portuguese:

• ficar com fome = to get hungry
• ficar com sono = to get sleepy
• ficar com medo = to become afraid

So ficar com often expresses a change of state.

It can, but estar/ficar com dor is a very natural and common way to talk about pain.

You could also hear:

• Se minhas costas doerem, vou descansar em casa.
• Se eu estiver com dor nas costas, vou descansar em casa.

But ficar com dor nas costas is especially natural when you mean if I start having back pain or if I end up with back pain.

So this sentence focuses on the pain appearing, not just existing.

Dor nas costas means back pain or pain in the back.

Breakdown:

• dor = pain
• nas = em + as
• costas = back

Even though costas literally also means backs or shoulders/back area, in this expression it simply means the back.

So:

• dor nas costas = pain in the back / back pain

This is the standard way to say it in Portuguese.

Because nas is a contraction of em + as.

• em as costas becomes nas costas

Portuguese uses contractions like this very often:

• em + o = no
• em + a = na
• em + os = nos
• em + as = nas

So dor nas costas is the normal contracted form.

That is just how Portuguese expresses this body part in many contexts. Costas is normally used in the plural when referring to a person’s back.

Examples:

• Estou com dor nas costas. = I have back pain.
• Ela machucou as costas. = She hurt her back.

So even though English uses singular back, Portuguese usually uses plural costas.

Yes, it can often be omitted because Portuguese verbs usually show the subject clearly:

• Se eu ficar com dor nas costas...
• Se ficar com dor nas costas...

Both can be correct depending on context.

But speakers often include eu for clarity, emphasis, or just natural rhythm. In a standalone sentence like this, keeping eu is very normal and helpful for learners.

Because in everyday Brazilian Portuguese, the structure ir + infinitive is extremely common for future meaning.

• vou descansar = I’m going to rest / I will rest

The simple future descansarei is grammatically correct, but it often sounds more formal, literary, or less conversational.

So in natural spoken Brazilian Portuguese:

• vou descansar em casa is the most normal choice

It can mean either one depending on context. In this sentence, it is basically expressing a future result:

• If I get back pain, I’ll rest at home.

Portuguese vou + infinitive often covers both:

• I’m going to...
• I will...

So you should not translate it too literally every time.

Portuguese does not need a separate word like then in this structure. The meaning is already clear from the if-clause plus the main clause.

• Se eu ficar com dor nas costas, vou descansar em casa.

English often says:

• If I get back pain, then I’ll rest at home.

But then is optional in English, and Portuguese normally just leaves it out.

Yes, but it changes the tone a little.

• Se eu ficar com dor nas costas, vou descansar em casa. = normal, everyday if
• Caso eu fique com dor nas costas, vou descansar em casa. = in case I get back pain / should I get back pain

Notice that with caso, you use the regular present subjunctive:

• caso eu fique

With se for future possibility, you use the future subjunctive:

• se eu ficar

Both are correct, but se is more common in everyday speech.

The difference is mainly about change versus state:

• ficar com dor nas costas = to get back pain, to start having back pain
• estar com dor nas costas = to have back pain, to be experiencing back pain

So:

• Se eu ficar com dor nas costas... focuses on the pain beginning or happening
• Se eu estiver com dor nas costas... focuses on the condition existing

Both are natural, but they are not exactly the same.

Yes. Very often that is the practical meaning.

• descansar = to rest
• em casa = at home

So vou descansar em casa naturally suggests I’ll stay home and rest or I’ll rest at home.

Portuguese does not need to explicitly say ficar em casa unless the idea of staying home is especially important.

Yes, the comma is standard and appropriate here because the sentence begins with a conditional clause:

• Se eu ficar com dor nas costas, vou descansar em casa.

The first part is the if-clause, and the comma separates it from the main clause.

In informal texting, people may omit it, but in correct writing, the comma is the normal choice.

It sounds natural and correct. A Brazilian speaker would understand it immediately.

Depending on style, a speaker might also say:

• Se eu ficar com dor nas costas, vou ficar em casa descansando.
• Se eu estiver com dor nas costas, vou descansar em casa.
• Se minhas costas começarem a doer, vou descansar em casa.

But your original sentence is completely normal and idiomatic.

In natural speech, some sounds may get reduced or linked together:

• Se eu may sound close to seu in fast speech, though it is still understood as se eu
• vou descansar may flow together smoothly
• em casa often sounds like ẽ casa, with a nasal vowel in em

A rough learner-friendly guide might be:

seh eh-ooh fee-KAR kõ DOHR naz KOS-tas, voh des-kãn-SAR ẽ KAH-zah

This is only approximate, but it can help you hear the rhythm.