Se o motorista estiver cansado, é melhor não pegar o volante.

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

• Se o motorista está cansado... = if the driver is tired (more like describing a present fact)
• Se o motorista estiver cansado... = if the driver is tired / if the driver happens to be tired

In this sentence, the idea is conditional and forward-looking: in the event that the driver is tired. That is why estiver is the natural choice.

The verb here is estar, and estiver is the future subjunctive form for ele/ela/você.

Here, se means if.

It introduces a condition:

• Se o motorista estiver cansado... = If the driver is tired...

This is one of the most common uses of se in Portuguese.

Be careful, because se can also mean other things in other contexts, such as a reflexive pronoun, but here it is simply if.

It can do both, depending on context.

• It can mean the driver in a specific situation.
• It can also mean the driver in a general, generic sense, like talking about any driver.

In warning-style sentences like this, Portuguese often uses the singular with the definite article to express a general idea:

• Se o motorista estiver cansado...
  = If the driver is tired...

This sounds natural in Portuguese even when the meaning is general.

Because motorista here is being treated as masculine.

Motorista is a noun that can refer to a man or a woman, but the adjective must agree with the actual person:

• o motorista cansado = the tired male driver
• a motorista cansada = the tired female driver

So in this sentence:

• o motorista estiver cansado

the masculine form is used because of o.

Yes, literally it means is better, but in this kind of sentence it often means:

• it’s better to...
• it’s best to...
• you shouldn’t... / it’s advisable not to...

So:

• é melhor não pegar o volante
  means something like
  it’s better not to get behind the wheel

This is a very common and useful structure in Portuguese:

• É melhor esperar. = It’s better to wait.
• É melhor não sair agora. = It’s better not to go out now.

Because Portuguese often does not use a dummy subject like English it.

English says:

• It is better not to drive.

Portuguese simply says:

• É melhor não dirigir.

So é melhor is a normal impersonal structure. There is no need to add a subject.

Yes, pegar often means to take, to get, or to grab, but in the expression pegar o volante it has an idiomatic meaning.

Here, pegar o volante means:

• to take the wheel
• to get behind the wheel
• to drive

So the sentence does not mean physically grabbing a steering wheel with your hand. It means assuming control of the car.

This is similar to English expressions like take the wheel or get behind the wheel.

In Portuguese, it is very common to use the definite article with nouns in expressions like this.

• pegar o volante = literally take the steering wheel

Even though English might sometimes say drive or get behind the wheel, Portuguese naturally keeps the article here.

So o volante is not unusual at all; it is the normal way to say it in this expression.

Yes. A very common alternative would be:

• Se o motorista estiver cansado, é melhor não dirigir.

That means essentially the same thing: If the driver is tired, it’s better not to drive.

The difference is mostly stylistic:

• dirigir = straightforward, direct, neutral
• pegar o volante = a bit more vivid, idiomatic, like get behind the wheel

Both are natural.

No. In standard neutral Portuguese, the negation goes before the infinitive:

• não pegar

So:

• é melhor não pegar o volante

is the correct form here.

Pegar não may appear in speech in certain emphatic contexts, but it is not the normal structure for this sentence.

Because after é melhor, Portuguese commonly uses an infinitive verb to say what is better to do or not do.

Pattern:

• É melhor + infinitive

Examples:

• É melhor esperar. = It’s better to wait.
• É melhor sair cedo. = It’s better to leave early.
• É melhor não pegar o volante. = It’s better not to get behind the wheel.

So pegar stays in the infinitive because it is part of that structure.

In informal spoken Portuguese, people do sometimes drop é and say:

• Melhor não pegar o volante.

That sounds natural in conversation.

However, in a full, neutral, standard sentence, especially for learners, it is better to keep:

• é melhor não pegar o volante

So the version with é is the safest and most standard one.

The sentence has two parts:

1. Se o motorista estiver cansado
   = conditional clause
   = If the driver is tired

2. é melhor não pegar o volante
   = main clause
   = it’s better not to get behind the wheel

So the overall pattern is:

• Se + condition, main statement

This is a very common pattern in Portuguese for giving advice, warnings, and instructions.

It is neutral and very natural Brazilian Portuguese.

It would sound fine in:

• safety campaigns
• advice
• everyday speech
• written instructions

It is neither especially formal nor especially slangy. The expression pegar o volante makes it sound a little more idiomatic than a very plain não dirigir, but it is still completely standard.