A buzina do autocarro assustou a criança, que estava a olhar para a chuva.

Because do is the normal contraction of de + o in Portuguese.

• de + o = do
• de + a = da
• de + os = dos
• de + as = das

So:

• a buzina do autocarro = the horn of the bus / the bus’s horn

Using de o autocarro would sound wrong in standard Portuguese.

It literally means the horn of the bus.

A native English speaker will often translate it more naturally as:

• the bus horn
• the horn of the bus
• the bus’s horn

In Portuguese, using de to show possession like this is very common.

Because Portuguese uses definite articles much more often than English.

So:

• a buzina = the horn
• a criança = the child
• a chuva = the rain

In English, we sometimes omit the, especially with things like rain, but Portuguese often keeps the article.

Assustou is the pretérito perfeito simples (simple preterite / simple past) of assustar.

Here it means:

• it frightened
• it scared

It is 3rd person singular, because the subject is a buzina do autocarro.

So the structure is:

• A buzina do autocarro = subject
• assustou = verb
• a criança = direct object

Because the verb assustou agrees with a buzina do autocarro, not with a criança.

The sentence structure is:

• A buzina do autocarro = the thing doing the frightening
• assustou = frightened
• a criança = the person who got frightened

So:

• The bus horn frightened the child

not

• The child frightened the bus horn

The comma shows that this is extra information about a criança.

So the sentence means:

• The bus horn frightened the child, who was looking at the rain.

This is a non-restrictive relative clause. It adds extra detail rather than identifying which child.

Without the comma, the meaning would be more like:

• the child who was looking at the rain

That would sound more restrictive, as if you are distinguishing that child from other children.

It refers to a criança.

So:

• a criança, que estava a olhar para a chuva
  = the child, who was looking at the rain

Grammatically, que can sometimes be ambiguous in Portuguese, just as who/which can be in English. But here the meaning makes it clear:

• a child can be looking at the rain
• a horn cannot normally be looking at the rain

So the relative clause clearly describes the child.

In European Portuguese, estar a + infinitive is the normal way to express an action in progress.

So:

• estava a olhar = was looking

This focuses on the ongoing action at that moment.

By contrast:

• olhava can mean was looking, but it is less explicitly progressive and can also suggest a more habitual or descriptive background meaning.

In this sentence, estava a olhar makes it very clear that the child was in the middle of looking at the rain when the horn sounded.

Because this sentence is in European Portuguese.

In European Portuguese, the usual progressive form is:

• estar a + infinitive
• estava a olhar

In Brazilian Portuguese, you would much more often hear:

• estar + gerúndio
• estava olhando

Both mean was looking, but estava a olhar is the normal Portugal Portuguese form.

Olhar para means to look at or to look towards.

So:

• olhar para a chuva = to look at the rain

Portuguese often uses para after olhar when describing the direction of the gaze.

Examples:

• olhar para o céu = to look at the sky
• olhar para a janela = to look at the window

Because the verb phrase here is olhar para.

So the pattern is:

• olhar para alguma coisa = to look at something

That is why the sentence has:

• para a chuva

not just

• chuva

Also, para a is not contracted here. Portuguese does not normally contract para + a in standard writing in the same way that de + a becomes da.

Not necessarily.

Criança is a grammatically feminine noun, so it takes:

• a criança
• not o criança

But it can refer to:

• a girl
• a boy
• a child of unspecified sex

So grammatical gender and real-life gender are not always the same thing.

Not in standard European Portuguese.

In Portugal, the usual word is:

• autocarro = bus

In Brazil, the usual word is:

• ônibus

So if you are learning Portuguese from Portugal, autocarro is the expected word.

Usually buzina refers to the horn itself, though by extension it can also suggest the horn sound.

In this sentence, a buzina do autocarro assustou a criança is understood naturally as:

• the bus horn frightened the child
• or more idiomatically, the sound of the bus horn frightened the child

Portuguese often allows this kind of shorthand, just like English does.

Yes, exactly.

That is one of the key grammar points in the sentence:

• assustou = completed event in the past
• estava a olhar = ongoing background action in the past

So the idea is:

• while the child was looking at the rain, the bus horn frightened them

This contrast between a finished event and an action in progress is very common in Portuguese storytelling and narration.