Đến tối nay, tôi sẽ trả lời email của bạn.

In this sentence, đến tối nay most naturally means by tonight.

• đến = to reach / until / by the time of
• tối nay = tonight / this evening

So Đến tối nay, tôi sẽ trả lời email của bạn means something like:

• By tonight, I will reply to your email
• I’ll reply to your email by tonight

It gives a time limit or deadline, not just a general future time.

Here, đến introduces a time point that the action will be completed by.

A very literal way to think of it is:

• đến tối nay = when it gets to tonight / by the time tonight comes

Vietnamese often puts time expressions at the start of the sentence to set the time frame first. That is very natural.

So the structure is:

• Đến tối nay = time frame
• tôi sẽ trả lời email của bạn = main statement

Sẽ is a common future marker. It tells you the action is in the future.

• tôi trả lời email của bạn = I reply / I am replying / I reply to your email
• tôi sẽ trả lời email của bạn = I will reply to your email

In this sentence, sẽ works like will in English.

Yes, it often can be omitted if the time expression already makes the future clear.

For example:

• Đến tối nay, tôi trả lời email của bạn.

This could still be understood as future because đến tối nay already sets a future time.

However, using sẽ makes the future meaning more explicit and often sounds more natural when you want to clearly promise or state an intention.

So:

• with sẽ = clearer will
• without sẽ = still possible, but slightly less explicit

Trả lời is a fixed verb meaning to answer or to reply.

• trả by itself often means return / pay
• lời means word / speech

But together, trả lời means reply. You should learn it as one unit.

Examples:

• trả lời câu hỏi = answer a question
• trả lời email = reply to an email
• trả lời bạn = reply to you

So in your sentence:

• trả lời email của bạn = reply to your email

Because here the direct thing being replied to is email của bạn.

• trả lời email của bạn = reply to your email
• trả lời bạn = reply to you

Both patterns are possible, depending on what you want to focus on:

• Tôi sẽ trả lời email của bạn. = I will reply to your email.
• Tôi sẽ trả lời bạn. = I will reply to you.

If you say trả lời cho bạn, it is understandable in some contexts, but trả lời bạn is usually the more natural basic pattern.

In this sentence, email của bạn most naturally means your email in the sense of the email/message you sent.

So:

• email của bạn = your email message

It does not usually mean your email address here.

If someone wanted to say your email address, they would normally make that clearer with extra words.

Vietnamese possession usually works like this:

• noun + của + possessor

So:

• email của bạn = your email
• nhà của tôi = my house
• sách của anh ấy = his book

This is different from English, where the possessor usually comes first: your email.

In Vietnamese, the thing possessed comes first, then the owner.

Vietnamese very often puts time expressions at the beginning of the sentence.

So this is extremely natural:

• Đến tối nay, tôi sẽ trả lời email của bạn.

It helps set the scene first: as for by tonight...

You could also say:

• Tôi sẽ trả lời email của bạn đến tối nay.

But this is much less natural for the meaning by tonight. Putting đến tối nay first is the normal and clearer choice here.

Yes, but the meaning changes a little.

• Tối nay, tôi sẽ trả lời email của bạn. = Tonight, I will reply to your email.
• Đến tối nay, tôi sẽ trả lời email của bạn. = By tonight, I will reply to your email.

So:

• tối nay focuses on when you’ll do it
• đến tối nay focuses on a deadline / completion by that time

That difference is important.

Yes. Email is very commonly used in everyday Vietnamese, especially in speech and modern writing.

You may also see more formal or older-style expressions such as:

• thư điện tử = electronic mail

But in normal usage, email is very natural.

You may also see it written as:

• email
• e-mail
• sometimes even imeo in very informal phonetic writing, though that is not standard

For learners, email is the safest everyday choice.

Not always. Vietnamese pronouns depend a lot on relationship, age, and politeness.

In this sentence:

• tôi = a polite, neutral I
• bạn = a fairly neutral you

This combination is common when speaking politely but not too formally, especially with someone of similar status or when the relationship is not very close.

Depending on who is speaking to whom, Vietnamese might instead use:

• mình
• anh / em
• chị / em
• cháu / cô
• and many others

So tôi ... bạn is natural, but it is not the only possible choice.

Vietnamese nouns usually do not change form for singular vs. plural.

So email could theoretically mean:

• email
• emails

But in this sentence, people will normally understand it as the email or your email message, because of the context.

If you wanted to make plurality clearer, Vietnamese could add other words, for example:

• những email của bạn = your emails

But without that, context does most of the work.

No. You do not need a classifier in this sentence.

Classifiers are often used when counting or specifying nouns:

• một email = one email
• hai email = two emails

But in trả lời email của bạn, the noun is just the object of the verb, so no classifier is needed.

That is completely normal.

It sounds natural, clear, and politely neutral.

• Đến tối nay gives a clear deadline
• tôi sẽ sounds like a promise or commitment
• trả lời email của bạn is standard modern wording

So this would work well in many everyday and professional situations, especially if you want to reassure someone that you will respond in time.