Na quinta-feira, eu vou responder ao e-mail do gerente.

Because Portuguese usually uses the preposition em for days when it means on, and here it combines with the feminine article a:

• em + a = na

So:

• na quinta-feira = on Thursday

Quinta-feira is treated as a feminine noun phrase because of feira.

You may also hear shorter, more casual versions in Brazil, such as:

• na quinta
• quinta-feira

But na quinta-feira is a very natural full form.

In Portuguese, the weekday names from Monday to Friday are traditionally written with a hyphen:

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

This is just the standard spelling. You should learn them as fixed expressions.

Literally, it means something like fifth fair/day of rest, but that literal meaning is not important in normal use. In modern Portuguese, it simply means Thursday.

The numbered weekday system works like this:

• segunda-feira = Monday
• terça-feira = Tuesday
• quarta-feira = Wednesday
• quinta-feira = Thursday
• sexta-feira = Friday

Then:

• sábado = Saturday
• domingo = Sunday

Because Na quinta-feira is an introductory time expression at the beginning of the sentence.

So the sentence is structured like:

• Na quinta-feira, = On Thursday,
• eu vou responder... = I’m going to answer...

This comma is very natural in writing. If the time phrase comes later, the comma is often not needed:

• Eu vou responder ao e-mail do gerente na quinta-feira.

Vou responder is the very common spoken way to express the future in Brazilian Portuguese.

It is formed with:

• ir in the present tense + infinitive

So:

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

There is also a simple future form:

• eu responderei

But in everyday Brazilian Portuguese, vou responder is much more common and natural in speech.

It can mean both, depending on context.

In natural English translation, it may be:

• I’m going to answer the manager’s email on Thursday
• I will answer the manager’s email on Thursday

Portuguese often does not make the same sharp distinction English sometimes does between going to and will.

Yes, it can be omitted.

Portuguese often drops subject pronouns when the verb already makes the subject clear:

• (Eu) vou responder ao e-mail do gerente.

Both are correct.

Including eu can add:

• emphasis
• clarity
• contrast

For example:

• Eu vou responder, mas ela não.
  I will answer, but she won’t.

Because the verb responder normally takes the preposition a in standard Portuguese when referring to the thing or person being answered.

So:

• responder a algo = to answer something
• responder a alguém = to answer someone

Since e-mail here is masculine and singular, a + o contracts to ao:

• a + o e-mail = ao e-mail

So:

• responder ao e-mail = to answer the email

In colloquial Brazilian Portuguese, many people do say:

• responder o e-mail

This is very common in everyday speech.

However, in more careful or traditional grammar, responder a is preferred:

• responder ao e-mail

So if you want the safer, more standard form, use responder ao e-mail.

Because Portuguese normally contracts certain prepositions with definite articles.

Here:

• a + o = ao

Other common contractions include:

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

So ao e-mail is the correct contracted form.

Borrowed words in Portuguese often end up being masculine by default, especially technology-related words.

So in Brazilian Portuguese, the common form is:

• o e-mail
• ao e-mail
• do e-mail

You may also see the spelling email without the hyphen. Both are common, though email is increasingly widespread.

Because do is a contraction of:

• de + o = do

Here, do gerente means:

• of the manager
• or, more naturally in English, the manager’s

So:

• o e-mail do gerente = the manager’s email / the email from the manager, depending on context

Most naturally here, learners usually understand it as the manager’s email.

Portuguese very often uses definite articles in places where English structure is different.

Here:

• o gerente = the manager
• do gerente = of the manager

This is the normal way to express possession or relationship:

• o carro do João = João’s car
• o escritório da médica = the doctor’s office

So o e-mail do gerente is a very normal Portuguese structure.

Yes, potentially.

Portuguese de can express different relationships, including:

• possession
• origin
• association

So o e-mail do gerente could mean:

• the manager’s email
• the email from the manager

Context usually tells you which meaning is intended.

Yes.

The sentence can also be:

• Eu vou responder ao e-mail do gerente na quinta-feira.

This means the same thing: On Thursday, I’m going to answer the manager’s email.

Putting Na quinta-feira at the beginning gives extra focus to the time.

No. In Brazilian Portuguese, the normal expression is:

• na quinta-feira

Using à quinta-feira would not be standard in this context.

So for on Thursday, stick with:

• na quinta-feira

Not always in every possible structure, but in expressions like this, na quinta-feira is the normal choice.

Compare:

• na quinta-feira = on Thursday
• quinta-feira can also appear in some contexts, especially as a heading, label, or shortened style

For a learner, na + weekday is a very reliable pattern.

It breaks down like this:

• Na quinta-feira = on Thursday
• eu = I
• vou responder = am going to answer / will answer
• ao e-mail = the email / to the email
• do gerente = of the manager / the manager’s

So the structure is:

time expression + subject + future with ir + infinitive + object

A very natural Brazilian Portuguese version would be:

• Vou responder ao e-mail do gerente na quinta-feira.

This is probably even more common in everyday speech than the full version with eu.

It is neutral and very natural.

• vou responder makes it sound like normal spoken Brazilian Portuguese
• ao e-mail is grammatically standard
• the whole sentence works well in both everyday and professional contexts

So it is neither especially slangy nor stiff.

You can follow the same model:

• na segunda-feira = on Monday
• na terça-feira = on Tuesday
• na quarta-feira = on Wednesday
• na quinta-feira = on Thursday
• na sexta-feira = on Friday

Examples:

• Na segunda-feira, eu vou ligar para ela.
• Na quarta-feira, vamos sair.
• Na sexta-feira, ele vai viajar.