Eu vou ao supermercado porque a promoção está boa.

Ao is a contraction of a + o.

• a = the preposition to
• o = the masculine singular definite article the

So:

• ir ao supermercado = to go to the supermarket

In Portuguese, this contraction is required:

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

Since supermercado is masculine and normally takes o, you get ao supermercado.

Yes. Vou ao supermercado and vou para o supermercado are both natural in Brazilian Portuguese.

A useful general idea is:

• ir a often sounds a bit more neutral or traditional
• ir para often emphasizes destination a little more

In everyday Brazilian Portuguese, para o supermercado is very common. So both are fine:

• Vou ao supermercado
• Vou para o supermercado

Both mean essentially the same thing here.

You can usually drop Eu.

Portuguese often leaves out subject pronouns when the verb already shows who the subject is. Since vou clearly means I go / I’m going, Eu is not necessary.

So these are both correct:

• Eu vou ao supermercado
• Vou ao supermercado

You would keep Eu if you want emphasis, contrast, or clarity:

• Eu vou ao supermercado, mas ele fica em casa.

Vou is grammatically present tense of ir:

• eu vou = I go / I’m going

In this sentence, it usually means I’m going or I’m going to the supermarket in a present or near-future sense.

Important: this is different from the future construction vou + infinitive, as in:

• vou comprar = I’m going to buy

But in your sentence, vou is the main verb by itself:

• Eu vou ao supermercado = I’m going to the supermarket

Here you use porque because it means because and introduces a reason.

• Eu vou ao supermercado porque a promoção está boa.

That is the standard form for giving an explanation.

A quick guide:

• porque = because
• por que = why / for what reason in questions or certain structures
• porquê = the reason (a noun)
• por quê = why, usually at the end of a sentence

So in this sentence, porque is the correct choice.

Because promoção is a feminine noun in Portuguese, so it takes a.

• a promoção
• uma promoção

Grammatical gender in Portuguese is something you usually have to learn with each noun. There is often a pattern, but not a guaranteed rule.

Many nouns ending in -ção are feminine:

• a promoção
• a situação
• a informação

So here, promoção is feminine, which also affects the adjective later: boa.

Because adjectives usually agree with the noun they describe.

Here, boa describes promoção, and promoção is feminine singular. So the adjective must also be feminine singular:

• a promoção está boa

Compare:

• o produto está bom
• a promoção está boa

So:

• bom = masculine singular
• boa = feminine singular

Portuguese often uses estar for temporary, current, or situational states, and ser for more permanent or defining characteristics.

Here, the sentence is talking about the current quality of the deal or sale, so está boa sounds natural:

• A promoção está boa. = The sale/deal is good right now.

If you said é boa, it could sound more like a general characteristic, depending on context. In this sentence, está is the more natural choice because promotions are temporary.

In this sentence, promoção most naturally means a sale, special offer, or deal.

So a promoção está boa means the current offer is good.

Be careful: promoção can also mean promotion in English, such as a job promotion. The meaning depends on context.

Examples:

• O produto está em promoção. = The product is on sale.
• Ela ganhou uma promoção no trabalho. = She got a promotion at work.

Portuguese uses definite articles much more often than English does.

So where English might say:

• I’m going to the supermarket because there’s a good sale
• or even just I’m going to supermarket in some varieties of English

Portuguese normally prefers the article:

• ao supermercado
• a promoção

This is very normal in Portuguese. In many cases, leaving the article out would sound unnatural.

Yes, grammatically you could, but it changes the feel a little.

• A promoção está boa = the deal is good right now, in this situation
• A promoção é boa = the deal is good, more like a general assessment

In everyday Brazilian Portuguese, está boa is very natural when talking about a current sale or offer.

The ending -ção is very common in Portuguese and is pronounced roughly like sown, but with a nasal sound instead of a clear n.

A rough approximation for English speakers is:

• pro-mo-SOWN

But that is only approximate. A few key points:

• ç sounds like s
• ão is nasal
• the stress is on the last syllable: -ção

So promoção sounds roughly like pro-mo-SOWN, with the final sound nasalized.

Yes, and they mean slightly different things.

• A promoção está boa = the sale/deal is good
• O supermercado está com promoção = the supermarket has a sale / is running a promotion
• Os produtos estão em promoção = the products are on sale

So your original sentence focuses on the quality of the deal, not just the existence of a sale. That is why a promoção está boa is a useful and natural choice.