Se chover, o lençol não vai secar na varanda.

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

So:

• Se chover, ... = If it rains, ...
• Se fizer sol, ... = If it is sunny, ...
• Se ele chegar cedo, ... = If he arrives early, ...

In this sentence, the rain has not happened yet, so Portuguese uses chover in the future subjunctive.

For the verb chover, the future subjunctive form happens to look exactly like the infinitive: chover.

It looks the same as the infinitive, but in this sentence it is functioning as the future subjunctive.

That is very common in Portuguese. For many verbs, some forms are identical in spelling. For example:

• infinitive: chover
• future subjunctive: chover

You can tell which one it is from the structure of the sentence. After se in a future condition, it is understood as future subjunctive.

Both are possible, but não vai secar is much more common in everyday Brazilian Portuguese.

• não vai secar = natural, common spoken future
• não secará = more formal, more written, less common in casual speech

So these are both grammatical:

• Se chover, o lençol não vai secar na varanda.
• Se chover, o lençol não secará na varanda.

In Brazil, the ir + infinitive future is usually the most natural choice in conversation.

In Portuguese, não normally comes before the verb it negates.

Here the verbal idea is vai secar = is going to dry / will dry, so the negation comes before vai:

• não vai secar = is not going to dry / will not dry

This is the normal position:

• não vai chover
• não posso
• não quero sair

Portuguese uses definite articles more often than English does.

So where English might say:

• the sheet
• or sometimes just sheet in a general household context

Portuguese often prefers:

• o lençol

Here o lençol means the sheet, referring to a specific sheet that is hanging outside.

Also, lençol is masculine singular, so it takes o.

Lençol usually means bed sheet.

In this sentence, it refers to a sheet that has been washed and put outside to dry.

A few useful related words:

• lençol = sheet
• fronha = pillowcase
• cobertor = blanket
• toalha = towel

So this sentence sounds like a very normal domestic situation: someone put a sheet outside, and if it rains, it will not dry.

Na is a contraction of em + a.

• em = in / on / at
• a varanda = the balcony / veranda
• em a varanda becomes na varanda

This kind of contraction is extremely common in Portuguese:

• no = em + o
• na = em + a
• nos = em + os
• nas = em + as

So:

• na varanda = on the balcony / in the veranda area

It can vary by context.

In Brazilian Portuguese, varanda often means a balcony, veranda, or a covered outdoor area attached to a home or apartment.

In this sentence, the most natural English translation is often balcony if we are imagining an apartment, or veranda/porch if we are imagining a house.

So na varanda is basically outside in that semi-open area of the home.

Yes. That is also correct.

Both of these are natural:

• Se chover, o lençol não vai secar na varanda.
• O lençol não vai secar na varanda se chover.

The first version puts more emphasis on the condition if it rains. The second starts with the main idea and adds the condition at the end.

Portuguese allows this kind of flexibility, just like English does.

Yes. Secar can be used both intransitively and transitively.

Intransitive:

• A roupa secou. = The clothes dried.

Transitive:

• O sol secou a roupa. = The sun dried the clothes.

In your sentence, it is intransitive:

• o lençol não vai secar = the sheet is not going to dry

So the sheet itself is the thing becoming dry.

It depends on what you want to say.

• não vai secar clearly refers to a future result: it will not dry
• não seca can sound more like a general fact, habitual situation, or a more immediate present-style statement

For this sentence, since it depends on a future condition (if it rains), não vai secar is the most natural everyday choice.

A sentence like Se chove, o lençol não seca na varanda is not the normal way to express this future conditional idea in Brazilian Portuguese.

A rough English-friendly pronunciation is:

len-SAWL

A few details:

• ç is pronounced like s
• the stress is on the last syllable: çol
• the final l in Brazilian Portuguese often sounds like a w sound, especially at the end of a syllable

So lençol sounds approximately like len-SAWw.

Also, chover sounds roughly like sho-VEHR in Brazilian Portuguese, and varanda roughly like vah-RAHN-da.

Because chover is an impersonal verb when it refers to weather.

Like English it rains, Portuguese simply says:

• chove = it rains
• vai chover = it is going to rain
• se chover = if it rains

Portuguese does not need a dummy subject like English it.

So you do not say:

• ele chove
• isso chove

You just say chove or chover depending on the structure.