Se fizer muito calor esta noite, vamos ligar o ar-condicionado em vez de abrir a janela.

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

• se fizer muito calor = if it gets / if it is very hot
• base verb: fazer
• future subjunctive form: fizer

This is very common in Portuguese:

• Se chover, ficamos em casa. = If it rains, we stay home.
• Se eu tiver tempo, eu vou. = If I have time, I’ll go.

English does not have this exact verb form, so it can feel unusual at first.

In Portuguese, weather expressions often use fazer in ways that do not translate word-for-word into English.

• fazer calor = to be hot
• fazer frio = to be cold

So:

• Faz calor hoje. = It’s hot today.
• Fez muito frio ontem. = It was very cold yesterday.
• Se fizer muito calor esta noite... = If it’s very hot tonight...

This is just a standard weather expression you need to learn as a chunk.

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

• esta noite = this evening / tonight
• hoje à noite = tonight

In many situations they mean almost the same thing.
Here, esta noite sounds natural and points to the upcoming night as a specific time period.

Examples:

• Vou sair esta noite.
• Vou sair hoje à noite.

Both mean I’m going out tonight.

In Brazil, hoje à noite is very common in everyday speech, but esta noite is also perfectly natural.

Both are possible, but vamos + infinitive is extremely common in spoken Brazilian Portuguese and often sounds more natural and conversational.

• vamos ligar o ar-condicionado = we’re going to turn on the air conditioning
• ligaremos o ar-condicionado = we will turn on the air conditioning

The simple future (ligaremos) is correct, but it can sound more formal, more written, or sometimes less common in everyday conversation.

So in normal speech, Brazilians often prefer:

• vamos abrir
• vamos sair
• vamos ligar

Ligar has several meanings, and this is a very common one.

Here it means:

• ligar um aparelho = to turn on a device

So:

• ligar o ar-condicionado = to turn on the air conditioner

Other common meanings:

• ligar para alguém = to call someone
• não ligar para isso = not to care about that / not to pay attention to that

Examples:

• Vou ligar a TV. = I’m going to turn on the TV.
• Vou ligar para minha mãe. = I’m going to call my mother.

Context tells you which meaning is intended.

Because ar-condicionado is a masculine noun phrase in Portuguese.

• o ar-condicionado = the air conditioner / the air conditioning

Even though English often says air conditioning as a general idea, Portuguese commonly uses o ar-condicionado as the thing/system/device.

Examples:

• O ar-condicionado está quebrado. = The air conditioner is broken.
• Liga o ar-condicionado, por favor. = Turn on the air conditioning, please.

The article o is very normal here.

Because ar-condicionado is the standard spelling for this compound noun in Portuguese.

It is treated as one lexical unit:

• ar-condicionado = air conditioner / air conditioning

You should just memorize it with the hyphen.

Examples:

• comprei um ar-condicionado
• o ar-condicionado está forte

Em vez de means instead of.

After em vez de, Portuguese normally uses:

• a noun: em vez de café, chá
• or an infinitive verb: em vez de abrir

So:

• em vez de abrir a janela = instead of opening the window

Examples:

• Em vez de sair, fiquei em casa. = Instead of going out, I stayed home.
• Em vez de comprar, alugamos. = Instead of buying, we rented.

This is the normal structure.

Portuguese usually uses articles more often than English does.

So even when English says open the window or sometimes just open a window depending on context, Portuguese often naturally says:

• abrir a janela

The article makes the phrase sound complete and idiomatic.

Other examples:

• fechar a porta = close the door
• lavar as mãos = wash your hands
• abrir os olhos = open your eyes

Using articles with everyday objects and body parts is very common in Portuguese.

Here it means we will / we’re going to turn on.

That is because it follows a condition:

• Se fizer muito calor esta noite, vamos ligar o ar-condicionado...

So the meaning is about what we are going to do if that condition happens.

But vamos + infinitive can also mean let’s ... in other contexts:

• Vamos sair! = Let’s go out!

So the structure can express either:

1. future intention/action, or
2. a suggestion/invitation

In this sentence, the conditional context makes the future meaning clear.

Yes, but it would sound a bit different.

• Se fizer muito calor esta noite... = If it gets very hot tonight...
• Caso faça muito calor esta noite... = In case it gets very hot tonight...

With caso, Portuguese usually uses the present subjunctive:

• caso faça

With se for future possibility, Portuguese uses the future subjunctive:

• se fizer

Both are correct, but se is more common and neutral in everyday speech.

Because muito is modifying the noun calor here.

• calor = heat
• muito calor = a lot of heat / very hot weather

In English, we usually translate fazer muito calor as to be very hot, but literally it is more like to make a lot of heat.

Compare:

• Faz calor. = It’s hot.
• Faz muito calor. = It’s very hot.

So muito intensifies the expression.

The original sentence is already natural. In everyday Brazilian speech, you might also hear slightly different versions such as:

• Se estiver muito quente esta noite, vamos ligar o ar-condicionado em vez de abrir a janela.
• Se fizer muito calor hoje à noite, a gente liga o ar-condicionado em vez de abrir a janela.

A few spoken-style differences:

• a gente instead of nós
• hoje à noite instead of esta noite
• estiver muito quente instead of fizer muito calor

All of these are natural in Brazil. The original sentence is clear, correct, and idiomatic.

A rough pronunciation guide for Brazilian Portuguese would be:

seh fee-ZEHR MOOY-too kah-LOR ES-tah NOY-chee, VAH-moos lee-GAR oo ar-kohn-dee-syo-NA-do em VEHS jeh ah-BREER ah zha-NEH-lah

A few notes:

• fizer has stress on the last syllable: fi-ZER
• calor also has final stress: ca-LOR
• noite sounds roughly like NOY-chee
• de in fast Brazilian speech often sounds like jee or djee before some sounds, so em vez de abrir may sound close to em vez d’abrir
• janela begins with the Portuguese j sound, like the s in measure

This is only an approximation, but it can help you hear the rhythm of the sentence.