O cachorro é rápido.

O is the masculine singular definite article, meaning the. It goes with cachorro because cachorro is a masculine singular noun.

• o cachorro = the dog
• um cachorro = a dog

So O cachorro é rápido means The dog is fast, not A dog is fast.

In Portuguese, many nouns have grammatical gender. Cachorro is the masculine form for dog, and cachorra is the feminine form.

• o cachorro = the male dog / the dog (masculine)
• a cachorra = the female dog

If you change the noun to the feminine form, the article and adjective usually change too:

• A cachorra é rápida.

Portuguese has two main verbs for to be: ser and estar.

Here, é is from ser, which is commonly used for characteristics seen as more essential, general, or defining.

• O cachorro é rápido. = The dog is fast.
  This sounds like being fast is a characteristic of the dog.

If you said O cachorro está rápido, it would sound more temporary or situational, like the dog is being fast right now or the dog seems fast at the moment. That is less common in a basic descriptive sentence.

É is the third-person singular form of ser in the present tense.

The present tense of ser is:

• eu sou = I am
• você / ele / ela é = you are / he is / she is
• nós somos = we are
• vocês / eles / elas são = you are / they are

So O cachorro é rápido literally follows the pattern:

• The dog + is + fast

It actually comes after the verb, not directly after the noun:

• O cachorro = the dog
• é = is
• rápido = fast

So the structure is very similar to English:
The dog is fast.

In Portuguese, adjectives can appear before or after nouns depending on the sentence, but after linking verbs like ser, they usually come after the verb:

• O carro é novo. = The car is new.
• A casa é grande. = The house is big.

Adjectives in Portuguese usually agree in gender and number with the noun they describe.

Since cachorro is masculine singular, the adjective is also masculine singular:

• cachorro rápido / O cachorro é rápido

If the noun were feminine singular, the adjective would change:

• A cachorra é rápida.

If plural:

• Os cachorros são rápidos.
• As cachorras são rápidas.

The accent in é shows both pronunciation and meaning.

1. Pronunciation: it marks the vowel as an open e sound.
2. Meaning: it distinguishes é (is) from e (and).

So:

• é = is
• e = and

This accent is very important because it changes the word.

The accent in rápido shows where the stress goes: RÁ-pi-do.

Without the accent, Portuguese stress rules would suggest a different pronunciation. The accent helps learners know that the first syllable is stressed.

So say it roughly like:

• RAH-pee-doo (very approximate)

The exact Brazilian Portuguese pronunciation is more natural than the English approximation, but the main point is that the stress is on the first syllable.

A rough English-friendly pronunciation is:

• kah-SHO-hoo or kah-SHOR-hoo, depending on accent

A few important sound notes:

• ca sounds like kah
• ch in Brazilian Portuguese usually sounds like sh
• rr in cachorro is not like the English r; in many Brazilian accents it sounds like a throaty h sound
• the final o is often pronounced like oo in everyday Brazilian speech

So cachorro does not sound like Spanish perro, and the rr is one of the trickier sounds for English speakers.

In many Brazilian accents, an unstressed final o sounds more like u.

So although it is written cachorro, it is often pronounced approximately like:

• cachorru

This is very common in Brazilian Portuguese. The spelling stays o, but the sound shifts in unstressed final position.

Sometimes yes, but in this sentence o is completely natural and expected.

Portuguese uses articles very often, often more than English does. So O cachorro é rápido sounds like a normal complete sentence meaning The dog is fast.

If you remove the article and say Cachorro é rápido, it sounds more general, like Dogs are fast or A dog is fast by nature, depending on context, and it is less likely to refer to one specific dog.

Yes. Rápido can often mean fast or quick, depending on context.

• fast is the most direct translation here
• quick can also work in many situations

The exact English word depends on what nuance you want, but the Portuguese word itself is very common and flexible.

You would make the article, noun, verb, and adjective plural:

• Os cachorros são rápidos. = The dogs are fast.

Changes:

• o → os
• cachorro → cachorros
• é → são
• rápido → rápidos

This is a good example of agreement in Portuguese: several words change together.

The normal order here is:

• O cachorro é rápido.

That is the most neutral and natural way to say it.

Portuguese can sometimes change word order for emphasis, poetry, or special style, but for a basic statement, stick with:

• article + noun + verb + adjective

So for learners, this sentence is a very good model of a standard Portuguese sentence.