Se eu estiver cansado, prefiro não pegar a estrada à noite.

Because se eu estiver cansado refers to a possible future condition: if I am / if I happen to be tired.

In Portuguese, when se means if and the condition is about the future, Portuguese usually uses the future subjunctive:

• Se eu estiver cansado... = If I am tired...
• Se ele chegar cedo... = If he arrives early...

You would not normally say se eu estou cansado in this kind of sentence.

• Se eu estou cansado... sounds like if I am tired right now, and it is much less natural here.
• Estiver is the correct form for this future/possible situation.

Estiver is the future subjunctive of the verb estar.

Here are the forms of estar in the future subjunctive:

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

This tense is very common after words like:

• se = if
• quando = when
• assim que = as soon as
• logo que = once/as soon as

when they refer to the future.

Because the sentence describes something uncertain or hypothetical in the future.

The speaker is not saying I am tired now. The speaker is saying:

• If I happen to be tired, then...

That uncertainty triggers the subjunctive.

A useful contrast:

• Se eu estiver cansado, prefiro não pegar a estrada à noite.
  If I’m tired, I prefer not to hit the road at night.

• Estou cansado.
  I am tired.
  This is a statement of fact, so no subjunctive.

Yes. Cansado is an adjective, so it changes according to the gender and number of the person being described.

• a man would say: cansado
• a woman would say: cansada
• a group of men / mixed group: cansados
• a group of women: cansadas

So a female speaker would normally say:

• Se eu estiver cansada, prefiro não pegar a estrada à noite.

Because não is negating the infinitive pegar.

• prefiro pegar a estrada = I prefer to hit the road
• prefiro não pegar a estrada = I prefer not to hit the road

This structure is very common in Portuguese:

• prefiro não sair = I prefer not to go out
• decidi não ir = I decided not to go
• tento não pensar nisso = I try not to think about that

So não comes directly before the infinitive verb it is negating.

Pegar a estrada is an idiomatic expression meaning something like:

• to hit the road
• to take the road
• to set out by road
• to drive/travel on the road

In this sentence, it suggests starting a road trip or driving on the highway/road, especially for some distance.

It does not literally mean physically grabbing a road. It is just an expression.

Similar natural English ideas would be:

• I prefer not to hit the road at night
• I prefer not to drive at night
• I prefer not to travel by road at night

Depending on context, any of those could fit.

Yes. A few common alternatives are:

• prefiro não dirigir à noite = I prefer not to drive at night
• prefiro não viajar à noite = I prefer not to travel at night
• prefiro não sair de carro à noite = I prefer not to go out by car at night

The difference is nuance:

• pegar a estrada suggests setting out on the road, often for a longer drive
• dirigir focuses on driving
• viajar focuses on traveling
• sair de carro focuses on going out by car

À noite means at night, and the à is a contraction of:

• a
  • a noite

This is called crase.

You should treat à noite as a fixed expression meaning at night.

Other time expressions work similarly:

• à tarde = in the afternoon
• à noite = at night
• às vezes = sometimes

Be careful not to confuse:

• a = to / the feminine singular article in some contexts
• à = contraction with crase

Yes, in Brazil de noite is also very common and natural.

• à noite = a bit more standard/written/formal
• de noite = very common in everyday speech

So these are both natural:

• prefiro não pegar a estrada à noite
• prefiro não pegar a estrada de noite

The meaning is basically the same.

Portuguese often omits subject pronouns when the verb already makes the subject clear.

So:

• Se eu estiver cansado, prefiro não pegar a estrada à noite.

is perfectly natural. The eu in the first clause already tells us who is speaking, and prefiro clearly means I prefer.

You could say:

• Se eu estiver cansado, eu prefiro não pegar a estrada à noite.

But the second eu is usually unnecessary unless you want emphasis or contrast.

Because prefiro expresses a general preference, not a one-time future action.

The meaning is:

• If I’m tired, my preference is not to drive/travel at night.

So the speaker is stating a general rule or habit. That is why the present tense sounds natural.

If you wanted to focus on a specific future occasion, you might say something different, but the original sentence is about a standing preference.

Yes, it sounds natural and correct.

It has a very normal structure:

• Se + future subjunctive for the condition
• prefiro não + infinitive for the preference
• à noite for time

Very natural Brazilian alternatives would be:

• Se eu estiver cansado, prefiro não dirigir à noite.
• Se eu estiver cansado, prefiro não pegar a estrada de noite.
• Se eu estiver cansado, é melhor eu não pegar a estrada à noite.

All are good, but the original sentence is completely idiomatic.