Na quarta-feira à tarde, eu vou responder ao e-mail da minha tia.

Because na is the contraction of em + a.

• quarta-feira is a feminine noun phrase, so it normally takes the article a
• em + a = na

So:

• na quarta-feira = on Wednesday

This is very natural in Brazilian Portuguese when talking about a specific day.

A few related forms:

• na segunda-feira = on Monday
• na terça-feira = on Tuesday
• na sexta-feira = on Friday

You may also sometimes hear just quarta-feira without na, depending on the structure, but in this sentence na quarta-feira is the standard form.

The names of weekdays in Portuguese are written with a hyphen:

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

This is simply the standard spelling. These are fixed compound words.

Also, unlike English, weekdays in Portuguese are normally not capitalized unless they begin a sentence.

So:

• quarta-feira = correct
• Quarta-feira = only if it starts the sentence

À is the result of a + a, and the accent marks this contraction. This is called crase in Portuguese.

In time expressions, Portuguese often uses:

• de manhã = in the morning
• à tarde = in the afternoon
• à noite = at night / in the evening

So à tarde is a fixed expression meaning in the afternoon.

Do not confuse:

• a = to / at / the feminine singular article in some contexts
• à = contraction of a + a

Yes, both are time expressions, but they answer different questions:

• na quarta-feira = when, in terms of the day
• à tarde = when, in terms of the part of the day

Together they mean:

• on Wednesday afternoon

Portuguese often combines these naturally:

• na segunda-feira de manhã = on Monday morning
• na quinta-feira à noite = on Thursday night
• na quarta-feira à tarde = on Wednesday afternoon

Yes, it can be omitted.

Portuguese is a pro-drop language, which means subject pronouns are often left out when the verb already makes the subject clear.

So both are correct:

• Na quarta-feira à tarde, eu vou responder ao e-mail da minha tia.
• Na quarta-feira à tarde, vou responder ao e-mail da minha tia.

Including eu may add:

• emphasis
• contrast
• clarity

For example, if you want to stress that I will do it, not someone else, eu makes sense.

Vou responder is the near future or periphrastic future:

• vou = I go / I am going
• responder = to answer

Together:

• vou responder = I’m going to answer / I will answer

In Brazilian Portuguese, this structure is extremely common in everyday speech.

There is also a simple future form:

• responderei ao e-mail da minha tia

That is grammatically correct, but it sounds:

• more formal
• more written
• less common in casual speech

So in natural spoken Brazilian Portuguese, vou responder is usually the more common choice.

Because the verb responder normally takes the preposition a when it means to answer something/someone.

So the structure is:

• responder a algo
• responder a alguém

Since e-mail here has the masculine article o, you get:

• a + o = ao

So:

• responder ao e-mail = to reply to the email

More examples:

• responder à pergunta = answer the question
• responder ao professor = answer the teacher
• responder aos clientes = reply to the clients

This is a very common point for English speakers, because English says simply answer the email, while Portuguese uses responder a.

The standard and most widely taught form is responder ao e-mail.

That is because, traditionally and grammatically, responder takes the preposition a:

• responder a uma mensagem
• responder ao e-mail
• responder à carta

You may occasionally hear forms without the preposition in informal speech, but for learners, responder ao e-mail is the safest and most correct choice.

Da is the contraction of de + a.

The phrase da minha tia literally means:

• of my aunt
• or more naturally in English, from my aunt / my aunt’s, depending on context

In this sentence, it shows possession:

• o e-mail da minha tia = my aunt’s email

Breakdown:

• de = of
• a = the
• minha tia = my aunt

So:

• de + a = da

Other examples:

• o carro da minha irmã = my sister’s car
• a casa da professora = the teacher’s house

Portuguese usually expresses possession with de rather than with an apostrophe-style structure like English.

So English:

• my aunt’s email

becomes Portuguese:

• o e-mail da minha tia

This is one of the most common ways to show possession in Portuguese.

Portuguese can also use possessives directly, like:

• meu livro = my book
• minha casa = my house

But when the possessor is a noun phrase like minha tia, Portuguese typically uses de:

• o e-mail da minha tia
• o nome do professor
• a bolsa da Maria

Because it agrees with tia, which is a feminine noun.

In Portuguese, possessives change according to the gender and number of the noun they describe:

• meu tio = my uncle
• minha tia = my aunt
• meus tios = my uncles
• minhas tias = my aunts

So here:

• tia is feminine singular
• therefore the possessive is minha

The phrase Na quarta-feira à tarde is an introductory time expression placed at the beginning of the sentence. The comma separates that introductory element from the main clause:

• Na quarta-feira à tarde, eu vou responder ao e-mail da minha tia.

This comma is natural and helpful in writing, especially because the time phrase is a bit long.

In some short sentences, punctuation can vary, and you may sometimes see it without a comma, but using the comma here is perfectly normal and clear.

Yes. Portuguese allows some flexibility in word order, especially with time expressions.

For example, these are all possible:

• Na quarta-feira à tarde, eu vou responder ao e-mail da minha tia.
• Eu vou responder ao e-mail da minha tia na quarta-feira à tarde.
• Vou responder ao e-mail da minha tia na quarta-feira à tarde.

The meaning stays basically the same, but the focus changes slightly.

• Putting Na quarta-feira à tarde first emphasizes the time
• Putting it later makes the action come first

E-mail is very common and widely understood in Brazil, but email without the hyphen is also seen more and more.

In practice:

• e-mail = very common
• email = also common in modern usage

Learners should recognize both. In this sentence, e-mail is perfectly natural.

It would become:

• Eu vou responder ao e-mail da minha tia.

That means simply:

• I’m going to answer my aunt’s email.

Adding Na quarta-feira à tarde gives the sentence a specific time frame. Without it, the action is still future, but not tied to a particular moment.

A rough pronunciation guide could be:

• Na kwar-ta-FAY-ra ah TAR-jee, eh-oo voh hehz-pon-DER ow ee-MAY-oo dah MEE-nyah TEE-ah

A few notes:

• quarta-feira: the quar sounds like kwar
• à tarde: in Brazilian Portuguese, d before e often sounds like j in many accents, so tarde may sound close to TAR-jee
• responder: the final r is often softer in Brazilian Portuguese than in English
• tia often sounds like CHEE-ah or TEE-ah, depending on accent and speed

This is only an approximation, but it can help you start hearing the sentence more clearly.