Se minha sobrinha me mandar um e-mail, eu vou responder ainda na terça-feira à noite.

Because after se in a real future condition, Portuguese normally uses the future subjunctive:

Se minha sobrinha me mandar um e-mail... = If my niece sends me an email...

For regular -ar verbs, the future subjunctive in the 3rd person singular has the same form as the infinitive, so mandar is correct here.

• mande = present subjunctive
• mandará = simple future

In this kind of if clause, mandará would not be the normal choice.

Me means to me.

So:

• minha sobrinha me mandar um e-mail = my niece sends me an email

It is the indirect object pronoun showing who receives the email.

This is the normal placement in Brazilian Portuguese: me mandar.

In addition, words like se often trigger proclisis, which means the object pronoun comes before the verb.

So Se minha sobrinha me mandar... sounds natural and standard.

You may also see mandar-me in very formal writing or in European Portuguese, but in Brazil me mandar is much more common.

Both are correct, but vou + infinitive is much more common in everyday Brazilian Portuguese.

• eu vou responder = very natural in speech
• eu responderei = correct, but more formal or literary

So this sentence sounds natural and conversational.

Yes. Portuguese often drops subject pronouns when the verb already makes the subject clear.

So both are possible:

• eu vou responder
• vou responder

Including eu can add a little emphasis or clarity, but it is not required.

In Brazilian Portuguese, possessives can appear with or without the definite article.

So both are possible:

• minha sobrinha
• a minha sobrinha

The version without the article is completely natural. The choice often depends on region, style, and personal preference.

Because e-mail is normally treated as a masculine noun in Portuguese:

• um e-mail
• o e-mail

You may also see email written without the hyphen, but e-mail is still very common.

Here ainda gives the idea of still / as early as / already by that time / on that very same day, depending on how you translate it.

In this sentence, it suggests that the speaker will reply before Tuesday is over, specifically on Tuesday night.

So ainda na terça-feira à noite has the sense of:

• still on Tuesday night
• that same Tuesday night

It adds emphasis that the reply will not be delayed beyond Tuesday.

Na is a contraction of em + a.

• em a terça-feira → na terça-feira

This is the normal way to say on Tuesday in this kind of sentence.

So:

• na terça-feira = on Tuesday

Because à is the contraction of a + a, marked with a grave accent.

• a
  • a noite → à noite

This is called crase.

In time expressions, this is very common:

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

Notice that de manhã is different and does not use à.

It can cover both, depending on context.

In Portuguese, noite often starts earlier than English night does, so terça-feira à noite can correspond to what English speakers might call Tuesday evening or Tuesday night.

Basically yes.

• na terça-feira à noite = fuller, more explicit
• terça à noite = shorter, very common in speech

Both mean on Tuesday night / Tuesday evening.

Because Portuguese often leaves the object unstated when it is obvious from context.

Here, the listener naturally understands that the speaker will respond to the email or to the niece.

If you wanted to make it explicit, you could say:

• vou responder ao e-mail
• vou responder a ela
• vou responder para ela (very common in Brazil)

Because the sentence begins with a conditional clause:

• Se minha sobrinha me mandar um e-mail, ...

After that opening if clause, Portuguese normally uses a comma before the main clause:

• ..., eu vou responder...

This is the standard punctuation.

The nh in sobrinha sounds like the ñ in Spanish niña, or roughly like ny in English canyon.

A rough English-friendly approximation is:

so-BREEN-ya

Not exact, but close enough to help.

So sobrinha does not sound like sob-rin-ha; the nh is a single sound.