A garçonete vai servir a sobremesa quando vocês terminarem o café.

Vai servir is the very common ir + infinitive future in Brazilian Portuguese. It works like is going to serve or simply will serve in English.

In everyday Brazilian Portuguese, this structure is often more natural than the simple future form servirá.

So:

• A garçonete vai servir... = very common in speech
• A garçonete servirá... = correct, but more formal or less conversational

Because the sentence is talking about something that will happen in the future.

After quando, Portuguese often uses the future subjunctive when the action has not happened yet:

• quando vocês terminarem o café = when you finish the coffee / when you have finished the coffee

If you used quando vocês terminam, it would usually sound more like a habitual or general situation, depending on context.

So the idea is:

• future event → quando vocês terminarem
• habitual/repeated event → often quando vocês terminam

Terminarem is the future subjunctive form of terminar for vocês/eles/elas.

In this tense, the forms are:

• eu terminar
• você/ele/ela terminar
• nós terminarmos
• vocês/eles/elas terminarem

Since the subject here is vocês, the correct form is terminarem.

In Brazilian Portuguese, você and vocês use third-person verb forms, not special second-person forms.

So:

• você termina
• vocês terminam
• vocês terminaram
• quando vocês terminarem

That may feel strange to an English speaker, because vocês means you all / you plural, but grammatically it behaves like they for verb agreement.

Here a is the definite article meaning the, not the preposition to.

• a sobremesa = the dessert

This happens because sobremesa is a feminine singular noun, so its article is a.

So in this sentence:

• servir a sobremesa = serve the dessert

You could also hear servir sobremesa, which is more general, like serve dessert as a category.
But servir a sobremesa sounds more like a specific dessert course that is expected in that situation.

In Portuguese, common nouns usually need an article or another determiner when they are the subject of a normal sentence.

So:

• A garçonete vai servir... = normal sentence
• Garçonete vai servir... = unusual in standard usage, unless it sounds like a headline, note, or some very special context

This is one reason Portuguese often uses articles more than English does.

In this sentence, o café most likely means the coffee that the people are drinking.

Depending on context, café can mean different things:

• coffee as a drink
• breakfast, in expressions like café da manhã
• sometimes a coffee break or café-related context

But here, with terminarem o café, the most natural reading is finish the coffee.

Yes. Portuguese allows that very naturally.

You can say:

• A garçonete vai servir a sobremesa quando vocês terminarem o café.
• Quando vocês terminarem o café, a garçonete vai servir a sobremesa.

Both are correct. The meaning stays the same.
The version with quando first may slightly emphasize the time condition.

Yes. A garçonete servirá a sobremesa quando vocês terminarem o café is grammatically correct.

The difference is mostly style:

• vai servir = more common in everyday Brazilian Portuguese
• servirá = more formal, more written, or more literary

So if you are aiming for natural spoken Brazilian Portuguese, vai servir is usually the better choice.

Yes, garçonete is a standard word for a female server or waitress.

A few notes:

• garçom = waiter
• garçonete = waitress
• atendente can be used in some places as a more neutral word, depending on the setting

In everyday interaction, Brazilians may also call a server with words like moço or moça, depending on the situation, rather than using the job title directly.