Minha orelha arde quando eu fico muito tempo na água da piscina.

Yes, both can relate to ear, but they are not always the same.

• orelha = the outer ear, the visible ear on the side of the head
• ouvido = the hearing organ / inner ear / sense of hearing

In everyday Brazilian Portuguese, people do sometimes use them loosely, but the distinction still matters. In this sentence, orelha arde suggests a physical burning or stinging sensation in the ear area. Depending on the exact situation, a Brazilian speaker might also say meu ouvido arde if the discomfort feels more internal.

Both are possible.

• minha orelha
• a minha orelha

In Brazilian Portuguese, possessives can appear with or without the definite article. The version without the article is completely natural here. The version with the article may sound a little more explicit or slightly more formal in some contexts, but both are correct.

Arder literally means to burn, but with body parts it often means:

• to sting
• to burn
• to feel irritated
• to smart

So Minha orelha arde does not necessarily mean the ear is on fire. It means there is an uncomfortable burning or stinging sensation.

Because the subject is minha orelha, which is singular.

• minha orelha arde = my ear burns/stings
• minhas orelhas ardem = my ears burn/sting

The verb has to agree with the subject in number.

The present tense is often used for things that happen generally or repeatedly.

So this sentence means something like:

• whenever I stay in pool water too long, my ear starts to sting
• this is something that tends to happen to me

It is not just about one single event right now. It describes a habitual or general situation.

That’s true: Portuguese often drops subject pronouns because the verb ending already shows the subject.

So both of these are possible:

• quando eu fico
• quando fico

In Brazilian Portuguese, using eu is very common and sounds natural. It can add clarity or just reflect normal spoken style. It is not required here, but it is perfectly fine.

Here ficar means to stay/remain.

• ficar muito tempo na água = to stay in the water for a long time

If you used estar, it would focus more on simply being in the water, not on remaining there over a period of time. Since the idea is duration, ficar is the better choice.

Muito tempo means a long time or too long, depending on context.

In this sentence, it means that the speaker remains in the pool water for an extended period. Portuguese often uses this structure directly:

• ficar muito tempo = to stay for a long time

You do not need a separate word for for here.

Na and da are contractions.

• na = em + a
• da = de + a

So:

• na água = in the water
• da piscina = of the pool

Altogether, na água da piscina means in the pool water.

Yes, you could, but the meaning changes slightly.

• na piscina = in the pool
• na água da piscina = in the pool water

The original sentence emphasizes the water itself as the thing causing the irritation. If you say na piscina, it can still be understood, but it is a little less specific.

Yes, it is natural and understandable.

A Brazilian speaker might also say similar versions such as:

• Meu ouvido arde quando eu fico muito tempo na água da piscina.
• Minha orelha começa a arder quando eu fico muito tempo na água da piscina.

The original sentence is good. The only thing that might change in real life is whether the speaker chooses orelha or ouvido, depending on where the discomfort is felt.