Depois de correr, meu corpo fica quente e meus braços ficam cansados.

Because depois de is a fixed pattern in Portuguese meaning after. After de, Portuguese normally uses the infinitive when the subject is general or the same as the main clause.

So:

• depois de correr = after running
• depois de comer = after eating
• depois de estudar = after studying

Using depois correr would sound incomplete.

After a preposition like de, Portuguese usually uses the infinitive form of the verb.

Here, correr stays in the dictionary form because it is not being conjugated for a subject:

• Depois de correr... = After running...

This is very common in Portuguese:

• antes de sair = before leaving
• sem falar = without speaking
• por fazer = to be done / left to do

Ficar + adjective is very common in Portuguese to express becoming, getting, or ending up in a state.

So here:

• meu corpo fica quente = my body gets warm/hot
• meus braços ficam cansados = my arms get tired

If you used está / estão, it would focus more on the state itself rather than the change into that state:

• meu corpo está quente = my body is warm/hot
• meus braços estão cansados = my arms are tired

Both are possible in some contexts, but ficar works especially well after an activity like running.

Because the verb agrees with the subject.

• meu corpo is singular, so: fica
• meus braços is plural, so: ficam

This is standard subject-verb agreement:

• o braço fica cansado
• os braços ficam cansados

Because quente is an adjective that does not change between masculine and feminine in the singular.

So you get:

• corpo quente
• mão quente
• água quente

But it does change in the plural:

• corpos quentes
• mãos quentes

So quente is already the correct singular form here.

Because cansados agrees with meus braços, which is masculine plural.

• braço = masculine singular
• braços = masculine plural

So:

• meu braço fica cansado
• meus braços ficam cansados

If the noun were feminine plural, you would use cansadas:

• minhas pernas ficam cansadas

In Portuguese, body parts can be expressed in different ways depending on the structure.

In this sentence, meu corpo and meus braços sound natural because you are directly describing your body and your arms.

Portuguese also often uses articles with body parts, especially in reflexive constructions:

• Lavei as mãos. = I washed my hands.
• Ele levantou o braço. = He raised his arm.

But in your sentence, using meu / meus is completely natural and clear.

Yes, you can say o meu corpo and os meus braços.

In Brazilian Portuguese, articles before possessives are common, but omitting them is also very normal. So both of these are possible:

• meu corpo fica quente
• o meu corpo fica quente

And:

• meus braços ficam cansados
• os meus braços ficam cansados

The version without the article is very natural and straightforward in Brazilian Portuguese.

It can mean either, depending on context.

In this sentence, after running, quente usually suggests warm or hot in the sense of body heat increasing. English might choose either one depending on tone.

So:

• meu corpo fica quente = my body gets warm / hot

Context decides the best translation.

Normally, no. In standard Portuguese, it is better to repeat the verb here because you have two coordinated clauses with different subjects:

• meu corpo fica quente
• meus braços ficam cansados

So the full sentence sounds more natural:

• meu corpo fica quente e meus braços ficam cansados

Leaving out ficam would sound incomplete in this structure.

Yes. A very natural alternative would be:

• Depois de correr, fico com o corpo quente e os braços cansados.

This literally means something like After running, I end up with my body warm and my arms tired, but in natural English it corresponds well to I get hot and my arms get tired.

Your original sentence is still correct; this version just shifts the focus more directly onto I.

Because Depois de correr is an introductory phrase at the beginning of the sentence.

The comma helps separate that opening time expression from the main statement:

• Depois de correr, meu corpo fica quente...

In Portuguese, especially in careful writing, this comma is natural and helpful. In very short informal writing, people sometimes omit commas, but here the comma is a good choice.