O casamento vai ser pequeno, mas muito bonito.

Vai ser means is going to be.

It is the very common Portuguese way to talk about the future:

• vai = goes / is going
• ser = to be

So:

• O casamento vai ser pequeno = The wedding is going to be small

This structure is:

• ir (to go) + infinitive

Very common examples:

• Vai chover. = It’s going to rain.
• Vou estudar. = I’m going to study.

Portuguese uses definite articles much more often than English.

So O casamento literally means the wedding, and in natural Portuguese it is very normal to include o here.

• o = the for a masculine singular noun
• casamento = wedding / marriage

In many cases, leaving out the article would sound less natural or would change the meaning.

Because they agree with casamento, which is a masculine singular noun.

• casamento is masculine singular
• so the adjectives must also be masculine singular:
  • pequeno
  • bonito

If the noun were feminine, the adjectives would change:

• A festa vai ser pequena, mas muito bonita.

This is standard adjective agreement in Portuguese.

There are two different things happening here:

1. bonito agrees with casamento, so it is masculine singular.
2. muito here is an adverb meaning very, so it does not change.

So:

• muito bonito = very beautiful
• muito pequena = very small
• muito interessante = very interesting

But when muito means much/many, it does agree:

• muito dinheiro = a lot of money
• muita gente = a lot of people
• muitos livros = many books

In your sentence, muito means very, so it stays muito.

Because pequeno and bonito are describing the subject through the verb ser.

This is the same idea as in English:

• The wedding will be small not
• The small will wedding

Here, ser links the subject (o casamento) to its qualities (pequeno, bonito). These are called predicate adjectives.

So the structure is:

• subject + verb + adjective

Ser is used because the sentence is describing an essential or defining characteristic of the event.

• ser = to be, for identity, characteristics, definitions
• estar = to be, for states, conditions, locations

A wedding being small and beautiful is presented here as what the wedding will be like, so ser is the natural choice.

Compare:

• O casamento vai ser pequeno. = The wedding is going to be small.
• O salão está bonito. = The venue looks beautiful / is beautiful right now.

Yes. That is correct.

• vai ser = going to be
• será = will be

Both are grammatical, but they feel a little different:

• vai ser is very common in everyday spoken Brazilian Portuguese
• será can sound a bit more formal, direct, or written

So in casual Brazilian Portuguese, vai ser is often the more natural choice.

Not necessarily. In this sentence, pequeno most likely means something like:

• small
• intimate
• with few guests
• simple in size

So um casamento pequeno often means a wedding with a limited number of people, not that the wedding is physically tiny.

Context matters, but this is a very common and natural use.

Mas means but.

It connects two contrasting ideas:

• small
• very beautiful

The idea is:

• the wedding may not be big,
• but it will still be very beautiful.

So mas works just like English but in this sentence.

A natural pronunciation would be approximately:

oo cah-zah-MEN-too vai zeh peh-KEH-noo, maz MUYN-too boh-NEE-too

A few helpful notes:

• casamento has stress on men
• pequeno has stress on que
• bonito has stress on ni
• muito in Brazil is often pronounced something like mũito / muito, depending on region
• mas is usually pronounced like maz before a voiced sound

Yes.

Casamento can refer to:

• the wedding event
• the marriage itself

In this sentence, because of vai ser pequeno, mas muito bonito, it clearly refers to the wedding ceremony/event, not the married relationship.

A marriage itself would not usually be described as small, so the context makes the meaning clear.