Se a reserva do restaurante estiver confirmada, podemos sair mais cedo.

Because Portuguese normally uses the future subjunctive after se when talking about a condition that may be true in the future.

So:

Se a reserva do restaurante estiver confirmada...
means a future possibility: if the reservation ends up being confirmed.

If you said Se a reserva do restaurante está confirmada, that would sound like you are checking a present fact, not a future condition. In most everyday situations for this sentence, estiver is the natural choice.

Estiver is the future subjunctive of the verb estar.

This form is very common in Portuguese after words like:

• se = if
• quando = when
• logo que = as soon as
• assim que = as soon as

Examples:

• Se ele vier, falamos.
• Quando eu tiver tempo, ligo-te.
• Assim que estiver pronto, aviso.

English does not have a direct equivalent form, so this often feels unfamiliar to English speakers.

Because confirmada agrees with a reserva, which is feminine singular.

• a reserva → feminine singular
• therefore confirmada → feminine singular

If the noun were masculine, you would use confirmado:

• o pedido está confirmado
• a reserva está confirmada

This is a very common agreement pattern in Portuguese.

It is close to a passive idea, but here estar + past participle usually describes a state or result, not the action itself.

So estiver confirmada means something like:

• be in a confirmed state
• have already been confirmed

Compare:

• Se a reserva estiver confirmada... → if the reservation is confirmed / in confirmed status
• Se a reserva for confirmada... → if the reservation is confirmed by someone

In many contexts both are possible, but estar + participle focuses more on the result.

Do is a contraction of de + o.

So:

• de + o = do

Literally, a reserva do restaurante means the reservation of the restaurant.
In natural English, that becomes the restaurant reservation or the reservation at the restaurant.

Portuguese often uses this noun + de + article + noun structure where English prefers a noun in front:

• a porta do carro = the car door
• o nome do hotel = the hotel’s name
• a reserva do restaurante = the restaurant reservation

Because Portuguese often uses the present tense to talk about the near future, especially when the situation feels practical, immediate, or already being considered.

So:

• podemos sair mais cedo = we can leave earlier / we’ll be able to leave earlier

This sounds natural and conversational.

Poderemos sair mais cedo is also grammatically correct, but it sounds a bit more formal, more distant, or more explicitly future-oriented.

In everyday European Portuguese, the present tense is very common for this kind of idea.

Mais cedo means earlier or sooner.

• cedo = early
• mais cedo = earlier

In this sentence, it means leaving before the usual or expected time.

Examples:

• Cheguei cedo. = I arrived early.
• Saí mais cedo. = I left earlier.

English speakers sometimes want to translate it word-for-word as more early, but Portuguese uses mais cedo where English uses earlier.

Yes. You can also say:

Podemos sair mais cedo, se a reserva do restaurante estiver confirmada.

or more commonly without the comma:

Podemos sair mais cedo se a reserva do restaurante estiver confirmada.

The meaning stays basically the same. The difference is mainly one of emphasis:

• Se... , podemos... puts the condition first
• Podemos... se... starts with the main idea

Both are natural.

When the se-clause comes first, the comma is standard and very natural:

Se a reserva do restaurante estiver confirmada, podemos sair mais cedo.

When the conditional clause comes after the main clause, the comma is often omitted:

Podemos sair mais cedo se a reserva do restaurante estiver confirmada.

So in your original sentence, the comma is exactly what you would expect.

Because Portuguese often uses the definite article when referring to a specific thing already understood in the context.

So a reserva suggests a particular reservation that both speaker and listener know about.

Compare:

• Temos uma reserva. = We have a reservation.
• A reserva está confirmada. = The reservation is confirmed.

English sometimes omits articles where Portuguese keeps them, so this is something learners need to get used to.

Yes. Reserva is the normal and natural word for a restaurant reservation in Portugal.

Examples:

• Tenho uma reserva para duas pessoas.
• A reserva está em nome de Silva.

A word like marcação is more often used for appointments or scheduled services, such as:

• marcação no dentista
• marcação de consulta

So for a restaurant, reserva is the best choice.

Sair is the most natural verb here.

Sair mais cedo is a very common expression meaning to leave earlier.

You could sometimes use ir embora, but it often sounds more like go away / head off, and in this kind of sentence sair mais cedo is the standard choice.

For example:

• Hoje vou sair mais cedo do trabalho. = Today I’m going to leave work early.

So for a learner, sair mais cedo is the phrase worth remembering.

Yes, if the context already makes it clear what reservation you mean.

So you could say:

Se a reserva estiver confirmada, podemos sair mais cedo.

That sounds completely natural if everyone already knows you are talking about the restaurant booking.

Including do restaurante just makes the sentence more explicit.