O cavalo corre rápido.

O is the masculine singular definite article, equivalent to the.

It matches cavalo, which is a masculine singular noun:

• o cavalo = the horse

In Portuguese, singular count nouns very often use an article in normal sentences, especially when they are the subject.

In this case, yes: cavalo is a masculine noun, so it takes o.

A useful beginner pattern is:

• many nouns ending in -o are masculine
• many nouns ending in -a are feminine

But this is only a pattern, not a perfect rule. You still need to learn the gender of each noun.

In Brazilian Portuguese, the article o is usually pronounced more like oo than like English oh.

So o cavalo often sounds roughly like:

• u ka-VA-lu

That is normal in speech.

Corre is the 3rd person singular present tense of correr.

So it matches subjects like:

• ele corre
• ela corre
• o cavalo corre

The infinitive is correr = to run.

Portuguese often leaves out subject pronouns when the verb form already makes the subject clear.

Here, corre already tells you it is third person singular, so o cavalo corre is completely natural.

You could say Ele corre rápido, but that would usually be for emphasis, contrast, or when the subject was already clear from context.

Often, yes, depending on context.

The simple present in Portuguese can talk about:

• habitual action
• general facts
• sometimes a current action, especially in narration

However, in Brazilian Portuguese, if you want to clearly say something is happening right now, people very often use:

• está correndo

So:

• corre = often runs
• está correndo = more clearly is running

Because Brazilian Portuguese very commonly uses an adjective form adverbially in everyday speech.

So:

• corre rápido = very natural
• corre rapidamente = also correct, but often more formal or more written

This happens with other words too:

• falar baixo
• andar devagar
• chegar cedo

So rápido here is normal and common.

No. In this sentence, rápido is describing how the horse runs, so it is functioning like an adverb.

That is why it stays in the default masculine singular form:

• corre rápido

Compare that with a true adjective modifying the noun:

• o cavalo rápido = the fast horse

There, rápido describes cavalo, not the action.

The accent mark shows that the stress goes on the first syllable:

• RÁ-pi-do

Without the accent, a learner might place the stress incorrectly.

So the accent helps you pronounce the word correctly and recognize its stress pattern.

In most Brazilian accents:

• the rr in corre is a strong, breathy or throaty sound, somewhat like an English h
• the initial r in rápido is usually pronounced the same strong way

So they do not sound like the usual English r.

Very rough approximations:

• corre ≈ KOH-hee
• rápido ≈ HA-pi-doo

The exact sound depends on the region, but for a beginner, thinking of it as a strong h-like sound is helpful.

Not really. It is a very basic, neutral sentence pattern:

• subject + verb + adverb
• O cavalo corre rápido.

This is a very common order in Portuguese and a good default pattern for beginners.

Yes. Um cavalo corre rápido is grammatical.

The difference is that:

• o cavalo refers to the horse
• um cavalo refers to a horse

So the grammar stays the same, but the article changes what kind of reference you are making.

Usually not in a normal sentence like this.

In Portuguese, bare singular count nouns are much less common than in English. So o cavalo or um cavalo sounds natural, while cavalo corre rápido sounds incomplete in ordinary speech.

You might see article-less nouns in very special contexts, like titles, labels, or headlines, but not as the normal beginner pattern.