Eu prefiro caminhar à noite quando a cidade está vazia.

Yes. In Portuguese, subject pronouns are often optional because the verb ending already shows who is doing the action.

So both of these are natural:

• Eu prefiro caminhar à noite quando a cidade está vazia.
• Prefiro caminhar à noite quando a cidade está vazia.

Including Eu can add emphasis, contrast, or clarity. For example, Eu prefiro... may sound like I prefer..., especially if you are comparing your preference with someone else’s.

Prefiro is the first-person singular form of the verb preferir in the present tense.

The verb forms are:

• eu prefiro = I prefer
• você/ele/ela prefere = you/he/she prefers
• nós preferimos = we prefer
• vocês/eles/elas preferem = you/they prefer

So prefiro is used because the subject is eu.

After preferir, Portuguese usually uses an infinitive to express the action that is preferred.

So:

• prefiro caminhar = I prefer to walk
• prefiro ler = I prefer to read
• prefiro ficar em casa = I prefer to stay home

This works much like English prefer + to + verb, except Portuguese often just uses the infinitive directly.

Yes, often you can, but there is a slight nuance.

• caminhar = to walk, often with a clearer sense of walking as an activity
• andar = to walk, but also more generally to go/move around

In many everyday contexts, Brazilians might naturally say andar:

• Prefiro andar à noite...

But caminhar can sound a bit more deliberate, like walking for pleasure, exercise, or calm movement. In this sentence, caminhar sounds very natural.

Because this is a contraction of:

• a = at/to
• a noite = the night

So:

• a + a noite = à noite

This grave accent is called a crase.

À noite is a fixed expression meaning at night / in the evening at night-time, depending on context.

A few similar examples:

• à tarde = in the afternoon
• à esquerda = on the left
• à meia-noite = at midnight

Yes. In Brazilian Portuguese, à noite and de noite can both mean at night, but they are not always identical in tone or usage.

• à noite is very common and often feels a bit more standard or neutral in many sentences.
• de noite is also common in speech and can sound slightly more conversational.

For this sentence, à noite is an excellent, natural choice.

Because this sentence expresses a habitual/general situation in the present:

• I prefer walking at night when the city is empty.

Here, quando means something like whenever or when, describing a repeated situation. So the present indicative works naturally:

• quando a cidade está vazia

If you were talking about a future situation, Portuguese often uses the future subjunctive:

• Quando a cidade estiver vazia, vou caminhar.
• When the city is empty / when the city becomes empty, I’ll go walk.

So:

• está = present, habitual/current description
• estiver = future/conditional kind of reference after quando

Portuguese usually uses estar for conditions or states that are seen as temporary, changeable, or situational.

• A cidade está vazia = The city is empty right now / at that time / in that situation

Using ser would suggest a more permanent or defining characteristic, which does not fit well here. A city is not inherently empty as part of its identity; it is empty at a certain time.

So está vazia is the natural choice.

In Portuguese, common nouns often need an article where English might not emphasize one as strongly.

• a cidade = the city

Here, the sentence is referring to the city as a known place in context, even if it is not named. That is very natural in Portuguese.

Just cidade without an article would usually sound incomplete here.

Because adjectives in Portuguese usually agree with the noun they describe in gender and number.

• cidade is a feminine singular noun
• so the adjective must also be feminine singular

Therefore:

• a cidade está vazia

Compare:

• o quarto está vazio = the room is empty
• a cidade está vazia = the city is empty
• as ruas estão vazias = the streets are empty

In Brazilian Portuguese, vazia is normally pronounced with a clear break between the i and the final a:

• va-ZI-a

So it has three syllables: va-zi-a.

That is useful because English speakers may want to compress it too much. Try to keep that final -ia clear.

Usually, no comma is needed here:

• Eu prefiro caminhar à noite quando a cidade está vazia.

That is the most standard punctuation.

The quando clause is closely tied to the main idea and identifies the circumstance of the preference. A comma could appear in some stylistic contexts for pause or emphasis, but normally you would write it without a comma.

Yes, Portuguese allows some flexibility, though the original order is very natural.

For example:

• Eu prefiro caminhar quando a cidade está vazia, à noite.
• À noite, eu prefiro caminhar quando a cidade está vazia.

But the original:

• Eu prefiro caminhar à noite quando a cidade está vazia.

sounds smooth, clear, and completely natural.

If you change the order, you may slightly shift the emphasis:

• À noite... emphasizes the time first.
• quando a cidade está vazia... emphasizes the condition more strongly.

Yes, it is very natural in Brazilian Portuguese.

Everything in it is standard and common:

• Eu prefiro...
• caminhar
• à noite
• quando a cidade está vazia

A Brazilian speaker would understand it immediately, and it sounds normal in both speech and writing.