Antes de passear, coloque a coleira no cachorro.

Because antes normally takes the preposition de before a verb in the infinitive.

So the pattern is:

• antes de + infinitive
• depois de + infinitive

Examples:

• Antes de sair, feche a porta.
• Depois de comer, lave a louça.

So antes de passear is the normal structure for before going for a walk / before taking a walk.

After antes de, Portuguese uses the infinitive form of the verb.

So:

• antes de passear = before going for a walk
• antes de comer = before eating
• antes de dormir = before sleeping

This is very common in Portuguese whenever a preposition is followed by a verb.

Here, passear means something like go for a walk or take a walk.

Depending on context, passear can mean:

• to stroll
• to go out for a walk
• to take someone/something for a walk

In this sentence, because of o cachorro and a coleira, the idea is clearly about going out with the dog.

A learner should also know that Brazilians often say:

• levar o cachorro para passear = to take the dog for a walk

So antes de passear here is a shorter, natural way to refer to that activity.

Coloque is the imperative form used with você.

So the sentence is giving an instruction:

• Coloque a coleira no cachorro. = Put the collar/leash on the dog.

In Brazilian Portuguese, commands often depend on which subject pronoun is being used:

• você → coloque
• tu → usually coloca in standard affirmative imperative, though real-life usage varies by region

Because você is very common in Brazil, coloque is a very normal instructional form.

It can sound:

• polite
• neutral
• instructional

So this sentence sounds like a normal instruction or reminder.

Yes, coloque is correct and natural, especially in instructions, signs, manuals, or polite speech.

But in everyday conversation, many Brazilians might also say:

• Põe a coleira no cachorro.
• Bota a coleira no cachorro.

These sound more casual and conversational.

Very roughly:

• coloque = more neutral/instructional/formal
• põe = very common, everyday
• bota = also very common in many regions, a bit more colloquial

So the original sentence is perfectly natural, but not the only way someone might say it.

Portuguese uses definite articles much more often than English.

So:

• a coleira = the collar
• o cachorro = the dog

Even when English might say something more generally, Portuguese often prefers the article when the item is understood from context.

Here, the sentence suggests:

• the collar that belongs to or is meant for the dog
• the dog already known in the situation

This is very normal Portuguese usage.

No is a contraction of:

• em + o = no

So literally:

• no cachorro = on the dog / onto the dog

This is a very common contraction in Portuguese:

• em + a = na
• em + os = nos
• em + as = nas

Examples:

• na mesa = on the table
• no carro = in the car
• nos livros = in the books
• nas caixas = in the boxes

In this sentence, colocar a coleira no cachorro means to put the collar on the dog.

Because the verb colocar usually focuses on where something is placed.

So:

• colocar algo em/no/na... = to place something on/in...

Here the collar is being physically put on the dog, so no cachorro is the natural choice.

Other prepositions would change the meaning:

• para o cachorro = for the dog
• ao cachorro would not sound right here for this idea

So:

• coloque a coleira no cachorro = put the collar on the dog

That is the standard structure.

Strictly speaking, coleira usually means collar.

That said, in pet-related contexts, learners sometimes see translations that use leash because the whole situation is about walking a dog. But the most direct meaning of coleira is collar.

If you specifically want leash, a very common word in Brazil is:

• guia

In some contexts, people may also use other regional or informal terms, but coleira itself is basically collar.

So this sentence is literally about putting the collar on the dog before the walk.

By itself, passear can be a little broad, but in this sentence the rest of the context makes the meaning clear.

Because it says:

• coloque a coleira no cachorro

the listener naturally understands that passear refers to going out with the dog.

If you wanted to make it more explicit, you could say:

• Antes de passear com o cachorro, coloque a coleira nele.
• Antes de levar o cachorro para passear, coloque a coleira nele.

Those versions are more specific, but the original sentence is already clear in normal context.

The comma separates the introductory phrase from the main command.

Structure:

• Antes de passear, = introductory time expression
• coloque a coleira no cachorro. = main clause

This is similar to English:

• Before going for a walk, put the collar on the dog.

The comma helps readability and is standard punctuation here.

Yes, but the structure changes slightly.

Compare:

• antes de passear = before going for a walk
• antes do passeio = before the walk

Both can work, but they are not exactly the same grammatically:

• antes de + verb
• antes de + noun → with article contraction, antes do if the noun is masculine singular

Examples:

• Antes de sair, apague a luz.
• Antes da viagem, organize as malas.

So antes do passeio is possible, but antes de passear is the version with a verb.

In Brazilian Portuguese, cachorro is extremely common in everyday speech and is usually the safest everyday word for dog.

• cachorro = common, everyday
• cão = also correct, but can sound a bit more formal, dictionary-like, or fixed-expression-like in some contexts

For a beginner in Brazilian Portuguese, cachorro is usually the more useful everyday choice.

So no cachorro sounds completely natural.