O veterinário vai castrar o gato.

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

• o veterinário = the veterinarian
• o gato = the cat

Portuguese uses articles very often, sometimes more often than English. In this sentence, both nouns are specific, so o is natural before each one.

Vai castrar is a very common way to talk about the future in Brazilian Portuguese.

It is built like this:

• vai = goes / is going
• castrar = infinitive, to neuter / to spay / to castrate

So vai castrar means is going to neuter or simply will neuter.

Portuguese also has a simple future form:

• O veterinário castrará o gato.

That is correct, but it sounds more formal or written. In everyday Brazilian Portuguese, vai + infinitive is much more common.

Historically, it comes from the idea of going to do something, but in modern Brazilian Portuguese, ir + infinitive usually just functions as a future construction.

So in this sentence, you should understand vai castrar as:

• is going to neuter
• will neuter

It does not usually emphasize physical movement.

Because the subject is o veterinário, which is third-person singular:

• eu vou = I am going to
• você/ele/ela vai = you/he/she is going to
• nós vamos = we are going to
• eles/elas vão = they are going to

Since o veterinário = he / the veterinarian, the correct form is vai.

In the structure ir + infinitive, only ir is conjugated. The main action verb stays in the infinitive.

So:

• vai castrar
• not vai castra
• not vai castrará

This is the same pattern you see in many Portuguese future expressions:

• Ela vai estudar.
• Nós vamos sair.
• Eles vão chegar cedo.

In everyday Brazilian Portuguese, castrar is commonly used for both male and female animals. So it can mean the general idea of having an animal fixed / neutered / spayed.

Even though English often distinguishes:

• neuter/castrate for males
• spay for females

Portuguese often just uses castrar for both.

Yes, grammatically gato is masculine singular and usually refers to a male cat.

• o gato = the male cat
• a gata = the female cat

However, in some contexts, masculine forms can also be used more generally. But if the speaker wants to clearly refer to a female cat, they would normally say a gata.

This is the normal Portuguese word order:

• subject: O veterinário
• verb phrase: vai castrar
• direct object: o gato

So it follows the common Subject + Verb + Object pattern, just like English:

• The veterinarian will neuter the cat.

Portuguese word order can be flexible in some contexts, but this sentence uses the most neutral and natural order.

The accent mark shows the stressed syllable.

• ve-te-ri-NÁ-rio

Without the accent, a learner might guess the stress incorrectly. In Portuguese, written accents often help show where the word is stressed and sometimes also indicate vowel quality.

So the important thing here is: the stress falls on ná.

A simple learner-friendly approximation is:

• veh-teh-ree-NAH-ryoo

The most important point is the stress:

• ve-te-ri-NÁ-rio

Depending on region, the sounds may vary a bit, but if you stress NÁ, you will already sound much more natural.

Sometimes, yes, but it depends on context.

Portuguese often allows the subject to be omitted when it is understood. However, vai can match several possible subjects:

• ele vai
• ela vai
• você vai
• o veterinário vai

So if you just say Vai castrar o gato, the listener will need context to know who is going to do it.

If the subject is not already clear, keeping o veterinário is better.

Yes. In more formal Portuguese, you could say:

• O veterinário vai castrá-lo.

Here, -lo means him/it and refers to o gato.

But in spoken Brazilian Portuguese, people often prefer other strategies, such as keeping the noun itself:

• O veterinário vai castrar o gato.

or using a more conversational pronoun structure, depending on context.

For a learner, the full noun version is the easiest and most natural place to start.

Yes.

• O veterinário = the veterinarian
  A specific veterinarian, known in the situation.
• Um veterinário = a veterinarian
  Any one veterinarian, not specifically identified.

So:

• O veterinário vai castrar o gato. = a specific vet will do it
• Um veterinário vai castrar o gato. = a vet will do it

Yes:

• o veterinário = the male veterinarian
• a veterinária = the female veterinarian

So if the vet is a woman, the sentence would be:

• A veterinária vai castrar o gato.

Notice that both the article and the noun change to feminine.

You could use other future-related forms, but they do not all sound equally natural here.

Most natural everyday options:

• O veterinário vai castrar o gato.
• O veterinário castrará o gato. (more formal)

Using está indo castrar would usually suggest literal movement, as if the vet is physically on the way to perform the procedure. That is a different nuance.

So for a simple future meaning, vai castrar is the best everyday choice.