Se eu ficar com sede, vou beber um pouco de água.

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

• Se eu ficar com sede = If I get thirsty
• Se eu fico com sede sounds wrong for this meaning

A simple way to remember it:

• se + future possibility → often future subjunctive
• se + general truth / repeated situation → often present tense

So in this sentence, the speaker is talking about a possible future situation, which is why ficar appears in the future subjunctive form.

It is the future subjunctive of ficar.

For ficar, the future subjunctive forms are:

• se eu ficar
• se você/ele/ela ficar
• se nós ficarmos
• se vocês/eles/elas ficarem

In this sentence, ficar does not mean to stay. It means to become:

• ficar com sede = to get thirsty
• ficar cansado = to get tired
• ficar doente = to get sick

In Portuguese, ficar often means to become or to get, not just to stay.

So:

• ficar com sede = to get thirsty
• literally, something like to become with thirst

This is a very common pattern in Portuguese. English often uses get where Portuguese uses ficar.

They are related, but not identical.

• ficar com sede = to get thirsty
• estar com sede = to be thirsty

So:

• Se eu ficar com sede, vou beber água. = If I get thirsty, I’ll drink water.
• Estou com sede. = I’m thirsty.

Ficar focuses on the change into that state. Estar describes the state itself.

Because estar com sede and ficar com sede are the normal, natural ways to express to be thirsty and to get thirsty in Portuguese.

There is an adjective sedento, but it is less common in everyday conversation and can sound more formal or literary in many contexts.

So a learner should strongly prefer:

• Estou com sede.
• Fiquei com sede.
• Se eu ficar com sede...

Both are correct, but vou beber is much more common in everyday Brazilian Portuguese.

• vou beber = very common, natural in speech
• beberei = correct, but more formal or less common in casual conversation

So these are both possible:

• Se eu ficar com sede, vou beber um pouco de água.
• Se eu ficar com sede, beberei um pouco de água.

In Brazil, the first version is what you are much more likely to hear.

Yes, that can happen in informal speech, because Portuguese sometimes uses the present tense to talk about a near-future result. But vou beber is clearer and more standard for a beginner.

Compare:

• Se eu ficar com sede, vou beber um pouco de água. = very clear future meaning
• Se eu ficar com sede, bebo um pouco de água. = possible in conversation, but less explicitly future

If you are learning, vou beber is the safest choice.

Because um pouco de means a little bit of or some.

This structure is:

• um pouco de + noun

Examples:

• um pouco de água = a little water / some water
• um pouco de café = a little coffee
• um pouco de tempo = a little time

So the de is required here.

Um pouco de água emphasizes that it is only a small amount.

• Vou beber água. = I’m going to drink water.
• Vou beber um pouco de água. = I’m going to drink a little water / some water.

So the longer version sounds a bit more specific and softer.

Yes, but it means something a little different in casual Brazilian Portuguese.

• beber água = to drink water in general
• beber um pouco de água = to drink a little water
• beber uma água = to have a water / drink some water

Uma água can sound colloquial and often refers to a serving of water, like ordering or asking for water. In your sentence, um pouco de água sounds more neutral and precise.

Yes, it could be omitted:

• Se eu ficar com sede, vou beber um pouco de água.
• Se ficar com sede, vou beber um pouco de água.

Both are possible.

However, Brazilian Portuguese often uses subject pronouns more than European Portuguese does, and keeping eu makes the subject very clear. For learners, including eu is often helpful and natural.

Yes.

You can say:

• Se eu ficar com sede, vou beber um pouco de água.
• Vou beber um pouco de água se eu ficar com sede.

Both are correct.

When the se clause comes first, it is usually followed by a comma. When it comes second, the comma is often omitted.

It is about a real possible future situation.

It does not mean something impossible or highly imaginary. It simply means:

• if that situation happens in the future, this will be the result

So this structure is very common for everyday conditional ideas:

• Se chover, vou ficar em casa.
• Se eu tiver tempo, ligo para você.
• Se eu ficar com sede, vou beber água.

After beber, the article is often omitted when talking about a substance in a general sense.

So these are common:

• beber água
• beber café
• beber leite

You may sometimes see beber a água, but that usually refers to a specific water already identified in context, such as the water on the table.

In your sentence, água means water in a general sense, so no article is needed.

The original sentence already sounds natural. But in casual speech, you might also hear:

• Se eu ficar com sede, vou tomar água.
• Se eu ficar com sede, eu bebo uma água.

A small note:

• beber água is completely correct
• tomar água is also very common in Brazil

So your sentence is natural, clear, and perfectly good Portuguese.