Se o suco estiver muito doce, eu vou colocar mais água.

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

So:

• Se o suco estiver muito doce... = If the juice is/turns out too sweet...

Using está would not be the standard structure for this kind of future condition.

It comes from estar.

Here, estiver is the future subjunctive form of estar.

A few forms are:

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

This tense is very common in Portuguese after words like se, quando, and assim que when the action is in the future.

Because the sweetness of the juice is being treated as a temporary condition, not a permanent characteristic.

• estar doce = to be sweet right now / in this situation
• ser doce = to be sweet by nature or as a general characteristic

So Se o suco estiver muito doce sounds natural here.

Portuguese uses definite articles more often than English.

So o suco literally means the juice, but in English we might just say juice if the context is clear.

Here, o suco refers to the specific juice being discussed.

Because muito here is an adverb, meaning very. It modifies the adjective doce, so it does not change form.

• muito doce = very sweet

Compare that with muito as an adjective before a noun:

• muita água = a lot of water
• muitos copos = many glasses

So:

• muito + adjective → no agreement
• muito + noun → agrees in gender and number

Because ir + infinitive is extremely common in Brazilian Portuguese, especially in everyday speech.

• eu vou colocar = I’m going to put / I will add
• eu colocarei = I will put / I will add

Both are correct, but:

• vou colocar sounds more natural in conversation
• colocarei sounds more formal or written

Yes.

You can say:

• Se o suco estiver muito doce, vou colocar mais água.

That sounds very natural.

Brazilian Portuguese often uses subject pronouns more than some other varieties of Portuguese, but if the subject is obvious, eu can be omitted.

Keeping eu can add a little emphasis or clarity.

Yes.

The verb colocar literally means to put, but in food and drink contexts it often means to add.

So colocar mais água is a natural way to say add more water.

Other possibilities:

• adicionar mais água = more formal
• pôr mais água = also very common and natural

Because it means an unspecified amount: more water.

In Portuguese, a bare noun is normal here:

• mais água
• mais açúcar
• mais sal

If you said mais da água, that would mean more of the water from some specific water already mentioned.

Because the sentence begins with a conditional clause:

• Se o suco estiver muito doce, ...

When this kind of clause comes first, a comma is standard.

If you reverse the order, the comma is often omitted:

• Vou colocar mais água se o suco estiver muito doce.

Both orders are correct.

Yes, but the meaning changes slightly.

• Se o suco estiver muito doce = if the juice is too sweet
• Se o suco ficar muito doce = if the juice becomes / ends up too sweet

So ficar focuses more on a change of state.

It sounds natural.

A Brazilian might say it exactly like that:

• Se o suco estiver muito doce, eu vou colocar mais água.

A slightly more casual version could be:

• Se o suco estiver muito doce, vou colocar mais água.

And another very natural alternative is:

• Se o suco estiver muito doce, vou pôr mais água.

All of these work well in Brazilian Portuguese.

A rough Brazilian Portuguese pronunciation is:

• Se o suco estiver muito doce, eu vou colocar mais água.
• see oo SOO-koo ees-chee-VEHR MOOY-too DOH-see, eh-ooh voh koh-loh-KAR maiz AH-gwah

A few helpful notes:

• suco sounds like SOO-koo
• estiver often sounds like ees-chee-VEHR in Brazil
• doce often sounds like DOH-see
• água sounds like AH-gwah

Pronunciation varies by region, but this will get you close.