Se eu estiver com dor de garganta, vou beber água com limão.

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

So:

• Se eu estiver com dor de garganta... = If I have / if I happen to have a sore throat...
• Se eu estou... sounds wrong here because estou is present indicative, and this sentence is about a future possibility, not a current fact.

This is a very common pattern in Portuguese:

• Se eu tiver tempo, vou. = If I have time, I’ll go.
• Se chover, fico em casa. = If it rains, I’ll stay home.
• Se ele vier, a gente conversa. = If he comes, we’ll talk.

So se + future subjunctive is one of the key things to learn for future if clauses in Portuguese.

Estiver is the future subjunctive form of estar for eu.

Here is the mini-conjugation:

• se eu estiver
• se você/ele/ela estiver
• se nós estivermos
• se vocês/eles/elas estiverem

It comes from the verb estar, which is irregular, so the form is not predictable from the infinitive in an obvious way.

You will see this same form in many common sentences:

• Quando eu estiver pronto... = When I’m ready...
• Se ela estiver cansada... = If she is tired...
• Onde você estiver... = Wherever you are...

Portuguese often uses estar com to talk about temporary physical states, symptoms, or conditions.

So:

• estar com dor de garganta = to have a sore throat
• literally, something like to be with throat pain

This pattern is extremely common in Brazilian Portuguese:

• Estou com fome. = I’m hungry.
• Estou com sono. = I’m sleepy.
• Estou com febre. = I have a fever.
• Estou com dor de cabeça. = I have a headache.

English usually says have, but Portuguese often says be with.

Both can exist, but they are used a little differently.

• dor de garganta is the standard expression for sore throat / throat pain as a condition or symptom.
• dor na garganta means pain in the throat, focusing more literally on the location.

In many everyday situations, dor de garganta is the more natural fixed expression when talking about the illness/symptom in general.

For example:

• Estou com dor de garganta. = I have a sore throat.
  This is the most normal everyday phrasing.

You may also hear:

• Estou com uma dor na garganta. = I have a pain in my throat.
  This can sound a bit more specific or descriptive.

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

• vou beber = near future / going to drink
• beberei = simple future / I will drink

In Brazil, the ir + infinitive structure is very common in speech:

• vou fazer
• vamos sair
• ele vai chegar

So this sentence sounds natural and conversational:

• Se eu estiver com dor de garganta, vou beber água com limão.

If you said beberei, it would sound more formal, more written, or sometimes slightly stiff in normal conversation.

Yes. Portuguese often drops subject pronouns because the verb ending already shows the subject.

So both are possible:

• Se eu estiver com dor de garganta...
• Se estiver com dor de garganta...

However, keeping eu is very normal and often helpful for clarity, emphasis, or rhythm. In a sentence like this, including eu sounds completely natural.

For learners, it is often easier to keep the pronoun until you get more comfortable.

Because dor de garganta works as a fixed expression for the symptom.

So Portuguese normally says:

• Estou com dor de garganta.
• Ela está com dor de cabeça.

Not usually:

• Estou com a dor de garganta in this context

Using a would usually sound specific in a different way, as if referring to a particular pain already known in context.

You can also sometimes hear:

• Estou com uma dor de garganta terrível.

Here uma is possible because the speaker is treating it as a terrible throat pain, emphasizing the experience. But the basic symptom expression is usually just dor de garganta.

Because limão here refers to lemon as an ingredient or flavor, not to a countable number of whole lemons.

So:

• água com limão = water with lemon / lemon water

Portuguese often uses the singular in this kind of ingredient phrase:

• chá com leite = tea with milk
• pão com queijo = bread with cheese
• arroz com feijão = rice and beans

If you specifically wanted to mention multiple lemons, you could, but that would be a different idea:

• água com dois limões = water with two lemons

Yes, but it may suggest something a little different depending on context.

• água com limão usually means plain water with lemon added
• água de limão can sound more like lemon-flavored water or a drink based on lemon

In many real-life situations, both might be understood, but água com limão is a very straightforward way to say water with lemon.

Both are possible, and both are common.

• beber água com limão
• tomar água com limão

In Brazilian Portuguese, tomar is often used for drinks, medicine, soup, and similar things, even though its basic meaning is broader than drink.

Examples:

• Vou tomar água.
• Ela tomou café.
• Preciso tomar um remédio.

So your sentence with beber is correct, but many Brazilians would also naturally say:

• Se eu estiver com dor de garganta, vou tomar água com limão.

Because the sentence begins with a conditional if clause:

• Se eu estiver com dor de garganta, vou beber água com limão.

That first part sets up the condition, and the second part gives the result. In Portuguese, it is standard to use a comma when the se clause comes first.

Compare:

• Se chover, não saio.
• Se eu tiver tempo, ligo para você.

If the order is reversed, the comma is often not used:

• Vou beber água com limão se eu estiver com dor de garganta.

So the comma here is normal and correct.

A rough pronunciation guide:

• água ≈ AH-gwa
• limão ≈ lee-MOWN
  The ending -ão is nasal, so it is not exactly like English own. The sound comes through the nose.

A few notes:

• The stress in água is on the first syllable: Á-gua
• The stress in limão is on the last syllable: li-MÃO
• The tilde in ã shows nasalization

If you want a rough full-sentence rhythm:

• Se eu estiver com dor de garganta, vou beber água com limão.

Brazilian Portuguese pronunciation will vary by region, but that stress pattern stays the same.

Yes. That is also possible.

Portuguese often uses either:

• Se eu estiver com dor de garganta, vou beber água com limão.
• Se eu estiver com dor de garganta, bebo água com limão.

The version with vou beber sounds clearly future and very conversational.

The version with bebo can sound more like:

• a habitual response
• a general rule
• what I do in that situation

So:

• vou beber = I’m going to drink
• bebo = I drink / I’ll drink, as a usual response

Both can work, but the original sentence sounds especially natural for a single future plan.

Yes, it is natural and correct.

A Brazilian might also say slightly different versions depending on style, for example:

• Se eu estiver com dor de garganta, vou tomar água com limão.
• Se eu ficar com dor de garganta, vou tomar água com limão.

But your original sentence is completely fine:

• Se eu estiver com dor de garganta, vou beber água com limão.

It sounds like a normal everyday sentence.