Se esta toalha estiver molhada, eu vou pegar a outra.

Because this sentence talks about a possible future situation: if this towel is wet.

After se meaning if, Portuguese often uses the future subjunctive when the condition refers to the future and is still uncertain. So:

• Se esta toalha está molhada... sounds wrong for this meaning.
• Se esta toalha estiver molhada... is the normal form.

A good shortcut is:

• if + future possibility → often se + future subjunctive

So estiver is used because the speaker does not know yet whether the towel will be wet.

Estiver is the future subjunctive of estar.

In this sentence, it means something like if it turns out to be or if it is/will be in a future situation.

Here are some forms of the future subjunctive of estar:

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

With se, it is very common:

• Se eu estiver cansado...
• Se ela estiver em casa...
• Se esta toalha estiver molhada...

Because molhada describes a temporary condition or state: the towel is wet right now, not permanently wet by nature.

In Portuguese:

• ser = more permanent characteristics, identity, classification
• estar = temporary states, conditions, location

So:

• A toalha está molhada. = The towel is wet.
• A toalha é macia. = The towel is soft.

Wetness is treated as a state, so estar is the natural choice.

Because molhada has to agree with toalha, and toalha is a feminine singular noun.

So:

• a toalha molhada
• o pano molhado
• as toalhas molhadas
• os panos molhados

Portuguese adjectives usually change form to match the noun in gender and number.

Yes, in many real-life Brazilian Portuguese situations, essa toalha would also sound natural.

Traditionally:

• esta = this, near the speaker
• essa = that, near the listener or previously mentioned
• aquela = that over there, farther away

But in everyday Brazilian Portuguese, the distinction between esta and essa is often weaker, and many speakers prefer essa in normal speech.

So:

• Se esta toalha estiver molhada... = correct
• Se essa toalha estiver molhada... = also very common in Brazil, depending on context

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

Compare:

• eu vou pegar = I’m going to take/get
• pegarei = I will take/get

The simple future (pegarei) is grammatical, but in normal spoken Brazilian Portuguese, the structure ir + infinitive is often preferred because it sounds more natural and conversational.

So this sentence could also be:

• Se esta toalha estiver molhada, pegarei a outra.

But the original version sounds more like everyday speech.

Here, pegar means something like take, get, or pick up.

So eu vou pegar a outra means the speaker will take the other towel instead.

In Brazilian Portuguese, pegar is a very common verb with several meanings depending on context, including:

• take / get / pick up
• catch
• grab
• sometimes other informal meanings depending on situation

In this sentence, it is simply the normal everyday way to say take/get the other one.

Because a outra means the other one.

Here, outra stands in for the other towel, and the article a helps make that clear.

Compare:

• a outra = the other one
• outra = another one / some other one

So:

• eu vou pegar a outra = I’ll take the other one
• eu vou pegar outra = I’ll take another one

In this sentence, a outra suggests there is a specific alternative towel available.

Yes. Portuguese often drops subject pronouns when the verb already makes the subject clear.

So both are fine:

• Se esta toalha estiver molhada, eu vou pegar a outra.
• Se esta toalha estiver molhada, vou pegar a outra.

Including eu can add a little emphasis or clarity, but it is not required.

Yes. Portuguese allows flexible word order here.

For example:

• Se esta toalha estiver molhada, eu vou pegar a outra.
• Eu vou pegar a outra se esta toalha estiver molhada.

Both mean the same thing. The first version puts the condition first, which is very common and natural.

No, not always.

It depends on the meaning of the sentence. In this sentence, the condition is about a future possibility, so future subjunctive is used:

• Se esta toalha estiver molhada...

But in other situations, se can be followed by other verb forms. For example:

• Se ela está em casa, eu não sei. = If she is at home, I don’t know.
• Se eu fosse rico... = If I were rich...
• Se chover, ficamos em casa. = If it rains, we stay home.

So the important point is not just se, but the kind of condition being expressed.

Yes, absolutely.

• Se esta toalha estiver molhada... = If this towel is wet...
• Se a toalha estiver molhada... = If the towel is wet...

Using esta makes the towel more specifically identified, as in this towel here. Without the demonstrative, the sentence is more neutral and simply refers to the towel already understood from context.