Quando o café acabar, eu vou ao mercado.

Because after quando when you are talking about a future event, Portuguese normally uses the future subjunctive.

So:

• Quando o café acabar = When the coffee runs out / is gone

For many regular verbs, the future subjunctive form looks exactly like the infinitive, so acabar here is not the plain infinitive in meaning, even though it looks the same.

Compare:

• Ele acaba = He/it ends
• Quando ele acabar = When he/it ends

This is one of the most important patterns in Portuguese:

• quando + future idea -> future subjunctive

Examples:

• Quando eu chegar... = When I arrive...
• Quando você puder... = When you can...
• Quando o café acabar... = When the coffee runs out...

Here, o café means the coffee.

In Portuguese, café can mean different things depending on context:

• coffee (the drink or the coffee supply)
• breakfast
• sometimes café / coffee shop

In this sentence, because of the verb acabar, the natural meaning is the coffee runs out or the coffee is gone.

So this sentence is about needing to buy more coffee, not about a coffee shop closing.

Here acabar means something like:

• to run out
• to be finished
• to be gone

So Quando o café acabar means When the coffee runs out.

This is very common with things you consume:

• A água acabou. = The water ran out.
• O açúcar acabou. = The sugar is gone.
• A gasolina acabou. = The gas ran out.

So even though acabar often literally means to finish / to end, in everyday use it often means to run out of something.

Yes, vou is the present tense form of ir and literally means I go. But in Portuguese, the present tense is often used to talk about a future action when the context makes the future clear.

In this sentence, the future meaning is already established by Quando o café acabar. Because of that, eu vou ao mercado is understood as:

• I’ll go to the market
• I’m going to the market

So Portuguese does not always need a special future form like English does.

You could also say:

• Quando o café acabar, irei ao mercado.

But irei sounds more formal or written. In normal Brazilian Portuguese, vou is very natural.

Ao is a contraction of:

• a + o = ao

Here the verb ir is being used with the preposition a:

• ir a algum lugar = to go to some place

So:

• vou ao mercado = I go / I’m going / I’ll go to the market

You will see this kind of contraction a lot in Portuguese:

• a + o = ao
• a + a = à
• de + o = do
• em + o = no

So ao mercado is just the normal contracted form.

Yes, you often can.

Both of these are possible:

• vou ao mercado
• vou para o mercado

Very generally:

• ir a can sound a bit more neutral or standard
• ir para emphasizes destination a little more

In everyday Brazilian Portuguese, both are common in many situations.
In this sentence, ao mercado sounds completely natural.

You may also hear no mercado after ir in casual speech in Brazil, but in careful standard grammar, ao mercado or para o mercado is safer.

Yes. Portuguese often drops subject pronouns when the verb form already shows who the subject is.

So these both work:

• Quando o café acabar, eu vou ao mercado.
• Quando o café acabar, vou ao mercado.

Both mean the same thing.

You might keep eu for:

• emphasis
• contrast
• extra clarity

For example:

• Quando o café acabar, eu vou ao mercado, não você.

But in a normal sentence, leaving out eu is very common.

Yes. You can also say:

• Eu vou ao mercado quando o café acabar.

That has the same basic meaning.

The version with the quando clause first is also very natural:

• Quando o café acabar, eu vou ao mercado.

A useful punctuation rule:

• If the time clause comes first, a comma is normally used.
• If it comes second, the comma is usually omitted.

So:

• Quando o café acabar, eu vou ao mercado.
• Eu vou ao mercado quando o café acabar.

Yes.

In Quando o café acabar:

• o café = the subject
• acabar = the verb

So literally, the structure is:

• when the coffee finishes / runs out

This is why the verb is in the singular form, matching o café.

If the subject changed, the verb form would change too. For example:

• Quando os cafés acabarem... = When the coffees run out...
• Quando as cápsulas acabarem... = When the capsules run out...

Because Portuguese and English organize future time differently.

In English, we say:

• When the coffee runs out, I’ll go to the market.

We do not normally say:

• When the coffee will run out...

Portuguese also avoids a plain future form after quando in this kind of sentence, but it uses the future subjunctive instead:

• Quando o café acabar...

So the pattern is:

• quando + future subjunctive
• main clause with present or future meaning

That is why Quando o café acabar, eu vou ao mercado is the natural Portuguese structure.