Se eu ficar com dor na perna, vou descansar em casa.

Because after se meaning if, Portuguese usually uses the future subjunctive when the condition refers to the future.

So in this sentence:

• Se eu ficar com dor na perna... = If I get pain in my leg / If my leg starts hurting...

For the verb ficar, the future subjunctive form is ficar, which happens to look exactly like the infinitive. That is why eu ficar may look odd to an English speaker at first.

A useful pattern is:

• Se eu fizer... = if I do
• Se eu tiver... = if I have
• Se eu puder... = if I can
• Se eu ficar... = if I get / if I become

So se eu ficar is the expected grammar here.

Here, ficar com means something like to end up with, to get, or to develop a temporary condition.

So ficar com dor means:

• to get pain
• to start having pain
• to end up sore

It does not mean literally to stay with pain in the English sense, even though ficar often means to stay or to remain.

Other common examples:

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

So this is a very common Portuguese pattern for entering a state.

Portuguese often expresses physical discomfort with com + noun.

So:

• estar com dor
• ficar com dor

are very natural ways to say someone has pain.

English often prefers an adjective or a verb, such as:

• my leg hurts
• my leg is sore

Portuguese can also use other structures, but com dor is extremely common and natural.

For example:

• Estou com dor de cabeça. = I have a headache.
• Ele ficou com dor nas costas. = He got back pain / his back started hurting.

So ficar com dor na perna is a very normal Brazilian Portuguese way to express that idea.

Na is a contraction of:

• em + a = na

So na perna literally means in the leg or on the leg, depending on context, but in natural English we usually say in my leg or just my leg hurts.

In Portuguese, this kind of location phrase is very common with body parts:

• dor na perna = pain in the leg
• dor no braço = pain in the arm
• dor na cabeça = pain in the head
• dor no peito = pain in the chest

So na perna simply marks the place where the pain is.

Because Portuguese often does not use a possessive like my with body parts when the owner is obvious from context.

So instead of saying:

• na minha perna

Portuguese very naturally says:

• na perna

Since the subject is eu, it is already clear whose leg is being talked about.

This is very common in Portuguese:

• Lavei as mãos. = I washed my hands.
• Ele levantou a cabeça. = He raised his head.
• Estou com dor no braço. = My arm hurts / I have pain in my arm.

You can say na minha perna if you want extra emphasis or contrast, but it is not necessary here.

Both are grammatically possible, but vou + infinitive is much more common in everyday Brazilian Portuguese.

So:

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

This is the normal spoken way to express the future.

The simple future:

• descansarei

is correct, but it sounds more formal, written, or less conversational in many situations.

In everyday Brazilian speech, people strongly prefer:

• vou descansar
• vai chover
• vamos sair
• eles vão chegar

So the sentence sounds very natural and modern.

Because descansaria means would rest, which usually suggests a more hypothetical or conditional idea.

In this sentence, the speaker is talking about a real possible future result:

• If that happens, I will rest at home.

That is why Portuguese uses:

• vou descansar or descansarei

not:

• descansaria

Compare:

• Se eu ficar com dor na perna, vou descansar em casa. = If I get pain in my leg, I’ll rest at home.
• Eu descansaria em casa, se tivesse tempo. = I would rest at home if I had time.

So descansaria belongs to a different type of conditional meaning.

Em casa means at home.

It is a fixed, very common expression used when talking about one’s home in a general sense.

So:

• vou descansar em casa = I’ll rest at home

Na casa means in the house or at the house, usually referring to a specific house.

Examples:

• Estou em casa. = I’m at home.
• Estou na casa da minha irmã. = I’m at my sister’s house.
• Tem muita gente na casa. = There are a lot of people in the house.

So in your sentence, em casa is exactly the natural choice.

Yes. That is also natural, and it is a bit more direct.

Compare the two:

• Se eu ficar com dor na perna... = If I get pain in my leg / if my leg starts hurting...
• Se minha perna doer... = If my leg hurts...

The version with ficar com dor focuses on entering that state.
The version with doer focuses more directly on the leg hurting.

Both are correct and natural in Brazilian Portuguese. The original sentence just uses a very common everyday structure.

They are close, but not always identical in nuance.

• dor na perna emphasizes the location of the pain: pain in the leg
• dor de perna can mean leg pain more generally, almost like naming the type of pain

In many everyday contexts, both can work. But dor na perna is especially natural when you are talking about a specific pain you feel in that body part.

Compare:

• Estou com dor na perna. = I have pain in my leg.
• Dor de perna é comum depois de correr. = Leg pain is common after running.

So the sentence’s choice, dor na perna, sounds very natural for a personal, specific situation.

Yes, very much so.

Ficar often suggests becoming or ending up in a state, often a temporary one.

So ficar com dor na perna suggests that the speaker may start feeling leg pain, not that this is necessarily a permanent condition.

That matches the rest of the sentence well:

• If I get pain in my leg, I’ll rest at home.

This sounds like a temporary reaction to a temporary problem.

If Portuguese wanted to talk about a more ongoing condition, it might use a different structure depending on context, but here ficar com is perfect for a possible short-term situation.

It is not always strictly necessary in Portuguese, because the verb form often shows the subject. But here eu is very natural and helpful.

You could hear:

• Se eu ficar com dor na perna...

Much more naturally than simply:

• Se ficar com dor na perna...

Why? Because in this kind of conditional sentence, saying eu makes the subject clear right away and sounds smoother.

Brazilian Portuguese uses subject pronouns more often than European Portuguese, so keeping eu here is completely normal.

Yes.

Descansar means to rest, so it is broader than to sleep.

Depending on context, it can mean:

• lying down
• taking it easy
• staying home and not doing much
• recovering
• possibly sleeping, but not necessarily

So vou descansar em casa means the speaker plans to stay home and rest, not specifically that they will go to sleep.