O cão da vizinha começou a latir assim que viu o focinho do nosso gato à janela.

Because da is the normal contraction of de + a.

• de = of / from
• a = the (feminine singular)

So:

• de a vizinha → da vizinha

This kind of contraction is extremely common in Portuguese:

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

So o cão da vizinha means the neighbour’s dog or more literally the dog of the neighbour.

Portuguese uses definite articles much more often than English.

Here, o cão means the dog, and that sounds completely natural in Portuguese because we are talking about a specific dog: the neighbour’s dog.

In English, we sometimes omit articles in places where Portuguese would not. In Portuguese, using o here is expected.

Compare:

• o cão da vizinha = the neighbour’s dog
• um cão da vizinha would sound different, more like one of the neighbour’s dogs

Since the sentence refers to a particular dog, o is the natural choice.

In European Portuguese, it is very common to use the definite article before possessives:

• o meu carro
• a tua casa
• o nosso gato

So o nosso gato is the normal European Portuguese way to say our cat.

In some other varieties, especially Brazilian Portuguese, the article may sometimes be omitted more often, but in Portugal the article is very standard and often expected.

So:

• o nosso gato = our cat

not because it is extra emphatic, but simply because that is normal grammar in European Portuguese.

After começar in Portuguese, the usual structure is:

• começar a + infinitive

So:

• começou a latir = started barking

This is the standard pattern.

Examples:

• começou a chover = it started raining
• comecei a estudar = I started studying
• começaram a rir = they started laughing

For an English speaker, this is worth remembering as a fixed pattern:
começar a fazer alguma coisa.

Latir is the verb used for what dogs do: to bark.

So:

• o cão começou a latir = the dog started barking

This is the normal verb for dog barking. It is a straightforward vocabulary item, but learners often want to check whether it is specifically for animals, and yes, it is.

You can think of it as the direct equivalent of to bark.

Assim que means as soon as.

It introduces an event that happens immediately after another one.

So the structure is:

• começou a latir assim que viu...
• started barking as soon as it saw...

It is a very common connector in Portuguese.

Other similar expressions include:

• logo que = as soon as
• mal = as soon as / hardly had... when... in some contexts

But assim que is one of the clearest and most common choices.

The subject is understood from context: it is still o cão da vizinha.

Portuguese often leaves out subject pronouns when the subject is clear. This is very normal.

So the sentence works like this:

• O cão da vizinha começou a latir
• assim que viu o focinho do nosso gato à janela

The understood subject of viu is still the neighbour’s dog.

In English, we often repeat the subject:

• The neighbour’s dog started barking as soon as it saw...

In Portuguese, repeating ele would usually be unnecessary here.

Viu is the preterite form of ver (to see), and it is used because this sentence describes completed events in the past.

• começou = started
• viu = saw

The preterite is the normal tense for a finished action or event in a narrative sequence.

Here the sequence is:

1. the dog saw something
2. the dog started barking

Both are viewed as complete events, so the preterite is the natural tense.

If the sentence were describing ongoing background circumstances, the imperfect might appear instead, but not here.

Focinho means snout / muzzle.

It is commonly used for the projecting nose-and-mouth area of an animal’s face. Even though English speakers may associate snout more strongly with pigs or dogs, Portuguese can use focinho for many animals, including a cat.

In this sentence, it suggests that the dog spotted just the cat’s nose/muzzle appearing at the window, which creates a vivid image.

So o focinho do nosso gato is not strange Portuguese. It is a natural and expressive way to refer to that part of the cat’s face.

This is another contraction:

• de + o → do

So:

• de o nosso gato → do nosso gato

This means of our cat.

The phrase breaks down like this:

• o focinho = the snout
• do nosso gato = of our cat

Together:

• o focinho do nosso gato = the snout of our cat

Portuguese uses these contractions constantly, so they are essential to get used to.

À janela literally contains the contraction:

• a + a → à

In this expression, à janela means something like:

• at the window
• by the window
• sometimes in the window

In context, it suggests the cat’s snout was visible at the window.

This expression is very common in Portuguese:

• estar à janela = to be at the window / standing at the window
• aparecer à janela = to appear at the window

So here, à janela tells us where the cat’s snout was seen.

Both à janela and na janela can relate to window, but they do not always feel the same.

In this sentence, à janela is idiomatic and natural for the idea of being at the window or showing at the window.

Very roughly:

• à janela often suggests at/by the window
• na janela more literally suggests in/on the window

Portuguese often uses à janela in situations where English would simply say at the window.

So this is not just a random preposition choice; it is part of a common pattern.

Grammatically, the dog saw o focinho specifically, not the whole cat.

That is important. The direct object of viu is:

• o focinho do nosso gato

So the sentence focuses on the cat’s snout appearing at the window, not on the entire cat.

This is one reason the sentence feels vivid: the dog reacts the moment even just that part of the cat becomes visible.

Because o focinho is not a person.

Portuguese sometimes uses a before a direct object when that object is a person or is personified in certain contexts, but that does not apply here.

So:

• viu o focinho = normal
• not viu ao focinho

There is nothing missing. O focinho is simply the direct object.

Yes, cão is a standard word for dog, and it is especially normal in European Portuguese.

Learners often know cachorro, but in Portugal:

• cão is very common as the general word for dog
• cachorro can also exist, but it often means puppy or is used differently depending on region and context

So in a sentence like this, o cão da vizinha sounds very natural in Portugal.

Yes. The structure is very natural:

• O cão da vizinha = subject
• começou a latir = main action
• assim que viu... = time clause
• o focinho do nosso gato à janela = what was seen and where

Portuguese word order is often similar to English in sentences like this, but Portuguese tends to package information a little more compactly, especially because it can omit subject pronouns and use contractions like da, do, and à.

So even if the sentence feels dense to a learner, its structure is quite normal and idiomatic.

Yes, but the meaning would shift slightly.

• assim que viu = as soon as it saw
• quando viu = when it saw

Assim que stresses immediacy. It tells you the barking began immediately after the dog saw the cat’s snout.

Quando is more neutral and simply situates the event in time.

So assim que is a better choice here if the point is the instant reaction.

Yes, it has a few features that fit European Portuguese very naturally:

• o nosso gato with the article before the possessive
• cão as a very standard word for dog
• começou a latir and the overall phrasing sound fully natural in Portugal
• à janela is also very idiomatic in European Portuguese

None of these makes the sentence incomprehensible elsewhere, but together they fit Portugal very well.