O cachorro está latindo.

O means the, so o cachorro means the dog.

In Portuguese, using the definite article is very common, often more common than in English.
So:

• o cachorro = the dog
• cachorro = dog / a dog / dogs depending on context

In this sentence, o cachorro refers to a specific dog.

Portuguese nouns have grammatical gender.

• o is the masculine singular definite article
• a is the feminine singular definite article

Cachorro is a masculine noun, so it takes o:

• o cachorro = the dog
• a cachorra = the female dog

Even when talking about animals, the grammatical gender of the noun matters.

This is the Portuguese way of expressing an action happening right now.

It uses:

• estar = to be
• latindo = barking

So:

• está latindo = is barking

This is called the present progressive or continuous form, similar to English is barking.

Structure:

• estar + gerund

Examples:

• está correndo = is running
• está comendo = is eating
• está dormindo = is sleeping

Portuguese has two verbs that can both mean to be: ser and estar.

Here, estar is used because the sentence describes a temporary action or state happening at the moment:

• O cachorro está latindo. = The dog is barking.

You would not use ser here.
É latindo would be incorrect.

A simple rule:

• ser = identity, permanent characteristics, time, origin, etc.
• estar = temporary states, location, ongoing actions

Latir is the infinitive, meaning to bark.

To form the gerund in Portuguese, verbs usually change like this:

• -ar → -ando
• -er → -endo
• -ir → -indo

Since latir ends in -ir, it becomes:

• latir → latindo

More examples:

• comer → comendo
• falar → falando
• abrir → abrindo

A rough pronunciation for an English speaker is:

oo kah-SHO-hoh es-TAH lah-CHEEN-doo

A few notes:

• cachorro has the stress on cho: ca-CHOR-ro
• In many Brazilian accents, ti before a vowel sounds like chee, so latindo may sound like la-CHEEN-do
• está is stressed on the last syllable: es-TÁ
• The final o in Brazilian Portuguese often sounds closer to oo

This is only an approximation, but it helps at first.

Because the subject is singular: o cachorro = the dog.

Portuguese verbs agree with the subject.

Here the subject is ele / o cachorro, so the verb is the third person singular form of estar:

• eu estou
• você/ele/ela está
• nós estamos
• eles/elas estão

Examples:

• O cachorro está latindo. = The dog is barking.
• Os cachorros estão latindo. = The dogs are barking.

Yes, but the meaning changes slightly.

• O cachorro está latindo. = The dog is barking right now.
• O cachorro late. = The dog barks / does bark / barks generally

So:

• está latindo focuses on the action in progress
• late can describe a habit, general truth, or simple present statement

If you want to emphasize that the barking is happening at this moment, está latindo is the better choice.

Yes, Portuguese often allows the subject to be omitted:

• Está latindo.

This can mean He/She/It is barking, depending on context.

However, Portuguese speakers often keep the subject when they want to make it clear who or what is doing the action.
So O cachorro está latindo is perfectly natural and clearer.

The plural would be:

• Os cachorros estão latindo. = The dogs are barking.

Notice all the changes:

• o → os
• cachorro → cachorros
• está → estão

The gerund latindo does not change.

In Brazilian Portuguese, cachorro normally means dog.

A few related words:

• cachorro = dog
• cachorra = female dog
• cão = dog, often a bit more formal or dictionary-like
• filhote de cachorro = puppy

In everyday Brazilian speech, cachorro is extremely common.

The normal word order is:

• Subject + verb + gerund

So:

• O cachorro = subject
• está latindo = verb phrase

This is very similar to English:

• The dog is barking.

You can change word order in Portuguese for emphasis in some contexts, but O cachorro está latindo is the standard, neutral order.