Ao chegar ao balcão, a passageira mostrou o passaporte e perguntou sobre o voo.

Ao chegar means upon arriving, when arriving, or more naturally in English, when she arrived.

This is a very common Portuguese structure:

ao + infinitive

It expresses the idea of when or upon doing something.

Examples:

• Ao sair, desligue a luz. = When leaving / As you leave, turn off the light.
• Ao entrar, ele sorriu. = When he entered, he smiled.

So in your sentence, Ao chegar ao balcão sets the scene first: When she arrived at the counter...

A more explicit alternative would be: Quando chegou ao balcão...

Both are correct, but ao + infinitive is compact and very common in written Portuguese.

Ao is the contraction of a + o.

• a = the preposition used here
• o = the for a masculine singular noun
• a + o = ao

So:

• chegar a o balcão → chegar ao balcão

This means to arrive at the counter.

You will see these contractions a lot in Portuguese:

• a + a = à
• a + o = ao
• de + o = do
• em + o = no

Since balcão is masculine singular, ao is the correct form here.

Good question. In more standard or careful Portuguese, chegar is traditionally used with a:

• chegar ao balcão
• chegar à escola
• chegar ao aeroporto

In everyday Brazilian Portuguese, many people also say chegar em:

• chegar no balcão
• chegar na escola

So:

• chegar ao balcão = more formal / standard / often preferred in writing
• chegar no balcão = very common in speech in Brazil

The sentence you have uses the more standard written form.

Here, balcão means counter or desk, especially a service counter.

In an airport context, it usually refers to something like:

• the check-in counter
• the service desk
• the airline counter

So ao balcão means she arrived at the place where an employee would help passengers.

Portuguese often uses definite articles more than English does.

So a passageira literally means the passenger.

In English, we sometimes omit the in certain contexts, but in Portuguese it is very normal to include it.

Also:

• passageira = female passenger
• passageiro = male passenger

So the sentence specifically tells you the passenger is female.

Both mostrou and perguntou are in the preterite tense, which is used for completed past actions.

• mostrou = showed
• perguntou = asked

This tense is appropriate because the sentence narrates specific actions that happened one after the other:

1. she arrived at the counter
2. she showed her passport
3. she asked about the flight

This is exactly the kind of situation where Portuguese uses the preterite.

Compare:

• mostrava / perguntava would sound more like repeated, ongoing, habitual, or background actions
• mostrou / perguntou gives clear completed events in a sequence

Portuguese uses articles much more often than English.

So:

• mostrou o passaporte = showed the passport
• perguntou sobre o voo = asked about the flight

Even when English might sound more natural without an article in some contexts, Portuguese often includes one.

This is especially common with:

• body parts
• possessions
• general references to a known thing in the situation

Here, the passport and the flight are understood as specific and relevant in the context, so the articles sound very natural.

Perguntou sobre o voo means asked about the flight.

Here, sobre means about.

So:

• perguntar sobre algo = to ask about something

This is a very natural way to express the topic of the question.

Depending on context, Portuguese can also use other patterns, such as:

• perguntou pelo voo
• perguntou qual era o horário do voo
• fez uma pergunta sobre o voo

But in your sentence, perguntou sobre o voo is a straightforward way to say that she asked for information related to the flight.

The comma separates the introductory clause from the main clause.

• Ao chegar ao balcão = introductory time expression
• a passageira mostrou o passaporte e perguntou sobre o voo = main action

This is similar to English punctuation in sentences like:

• When she arrived at the counter, the passenger showed her passport...

So the comma helps show that the first part sets the time/background for what follows.

Because it is still the same subject: a passageira.

Portuguese often avoids repeating the subject when it is already clear.

So:

• a passageira mostrou o passaporte e perguntou sobre o voo

means:

• the passenger showed the passport and asked about the flight

It would be possible to repeat the subject, but it would usually sound unnecessary:

• A passageira mostrou o passaporte e a passageira perguntou...

That sounds repetitive unless you want special emphasis.

This is a spelling point that often confuses learners.

The word is:

• voo = flight

Older spelling used to be vôo, but after the spelling reform, the accent was removed.

So modern standard Portuguese writes:

• voo
• enjoo
• perdoo

In pronunciation, voo is typically said in two syllables in Brazilian Portuguese: VO-o.

So even though it looks unusual, voo is the correct current spelling.