Na sexta-feira, eu vou ao mercado depois do trabalho.

Na is a contraction of em + a, and it means on the here.

• em = in/on/at
• a = the feminine singular article

Because sexta-feira is feminine, em a sexta-feira becomes na sexta-feira.

So:

• na sexta-feira = on Friday

This is the normal way to say on Friday in Portuguese.

It is feminine because the core noun is feira, which is a feminine noun.

The weekday names from Monday to Friday are built with an ordinal-like word plus feira:

• segunda-feira
• terça-feira
• quarta-feira
• quinta-feira
• sexta-feira

Since feira is feminine, you get:

• a sexta-feira
• na sexta-feira

Because weekday names like segunda-feira through sexta-feira are normally written with a hyphen in Portuguese.

So the correct spelling is:

• sexta-feira

not:

• sexta feira

Even though vou is present tense, Portuguese often uses the present to talk about a planned or scheduled future action, especially when there is a time expression.

Here, Na sexta-feira makes the future meaning clear:

• Na sexta-feira, eu vou ao mercado... = On Friday, I’m going to the market...

This works a lot like English:

• On Friday, I go to the market after work can sound future if the context is clear, though English more often prefers I’m going.

The verb ir can be used with different prepositions, and both are possible in many situations.

In this sentence, ir a is very standard:

• vou ao mercado

Here:

• a + o = ao

So ao mercado literally comes from a o mercado.

Vou para o mercado is also possible and natural in Brazilian Portuguese, but vou ao mercado is very common and often sounds a bit more standard or neutral.

A useful quick guide:

• ir a = common, standard, often used for destination
• ir para = also very common, often emphasizes going toward/to that place

Ao is a contraction of a + o.

• a = to
• o = the

So:

• ao mercado = to the market

Portuguese contracts many preposition + article combinations, and this is one of the most common ones.

Other examples:

• de + o = do
• em + a = na
• em + o = no

Because do is the contraction of de + o.

• depois de = after
• o trabalho = the work / workday / job

So:

• depois do trabalho = after work

Portuguese very often uses the article with nouns where English might not:

• depois do trabalho
• antes do almoço
• gosto do café

Even though English says after work, Portuguese naturally says after the work/workday in this structure.

You can leave it out.

Portuguese often drops subject pronouns because the verb ending already shows who the subject is.

• (Eu) vou ao mercado depois do trabalho.

Both are correct:

• Eu vou ao mercado depois do trabalho.
• Vou ao mercado depois do trabalho.

Including eu can add emphasis, contrast, or clarity.

For example:

• Eu vou ao mercado, mas ele vai à farmácia.

Yes. Portuguese word order is fairly flexible, especially with time expressions.

The original sentence:

• Na sexta-feira, eu vou ao mercado depois do trabalho.

Other natural possibilities:

• Eu vou ao mercado depois do trabalho na sexta-feira.
• Depois do trabalho, eu vou ao mercado na sexta-feira.
• Na sexta-feira, depois do trabalho, eu vou ao mercado.

The original version is very natural because it starts with the broad time frame Na sexta-feira and then adds depois do trabalho.

Yes. In everyday Brazilian Portuguese, na sexta is very common and natural.

So these both work:

• Na sexta-feira, eu vou ao mercado...
• Na sexta, eu vou ao mercado...

Na sexta-feira is a bit fuller and slightly more formal or explicit. Na sexta sounds more conversational.

Not always, but it is a very common and natural choice.

Mercado can mean:

• a market
• a grocery store
• a supermarket, depending on context

Depending on the situation, a Brazilian speaker might also say:

• supermercado = supermarket
• feira = street market / open-air market

So:

• vou ao mercado is perfectly natural
• but if you specifically mean a supermarket, vou ao supermercado may be even more precise

Yes, many Brazilians do say vou no mercado in everyday speech.

However, if you are learning standard grammar, vou ao mercado is the safer form to use.

So:

• vou ao mercado = standard and widely accepted
• vou no mercado = very common in informal spoken Brazilian Portuguese, but often avoided in formal teaching

For a learner, vou ao mercado is the best choice.