kyouzyuu ni tomodati no meeru ni henzi wo simasu.

今日中 literally means within today.

• 今日 = today
• 中 = inside / within

So 今日中に means by the end of today or sometime before today is over. It emphasizes a deadline, not just a general time.

The に marks the time limit or deadline for the action.

So:

• 今日に would not be natural here
• 今日中に means by today / before today ends

This is different from a simple time word like 今日, which often appears without a particle:

• 今日返事をします = I’ll reply today.
• 今日中に返事をします = I’ll reply by the end of today.

The second one sounds more like a promise or deadline.

Here, 友達のメール means my friend’s email or the email from my friend.

The particle の connects 友達 and メール, showing a relationship between them. In this sentence, the natural interpretation is:

• an email from a friend
• my friend’s email

If you wanted to say an email to my friend, Japanese would usually say something like:

• 友達へのメール
• 友達に送るメール

So in this sentence, the speaker is replying to an email that the friend sent.

That に marks what the reply is directed to.

In Japanese, 返事をする often uses に for the thing or person receiving the reply:

• メールに返事をする = reply to an email
• 手紙に返事をする = reply to a letter
• 友達に返事をする = reply to a friend

So メールに means to the email in the sense of in response to the email.

In natural English, we say reply to an email, so this matches pretty well.

Because 返事をする is the standard expression for to reply / to answer.

• 返事 = reply, answer
• する = do
• 返事をする = to give a reply / to reply

By contrast, 返す usually means to return something:

• return a book
• give something back

It can sometimes mean to reply back in certain contexts, but for learners, 返事をする is the safer and more common expression here.

So:

• 返事をします = I will reply
• 返します more often suggests I’ll return it

Because 返事をする is a noun + する pattern.

In Japanese, many actions are expressed as:

• noun + をする

Here:

• 返事 = reply
• 返事をする = to make/give a reply

The particle を marks 返事 as the thing being done.

This is very common in Japanese:

• 勉強をする = study
• 電話をする = make a phone call
• 返事をする = reply

Yes, most likely it means I will reply, but the subject is omitted.

Japanese often leaves out the subject when it is clear from context. So this sentence could literally be understood as:

• (I) will reply to my friend’s email by today

Depending on context, it could also be we, he, or she, but in everyday conversation, a polite sentence ending in します often refers to the speaker’s own action.

Because Japanese normally puts the main verb at the end.

This sentence follows a very typical Japanese order:

• 今日中に = by today
• 友達のメールに = to my friend’s email
• 返事を = a reply
• します = do / will do

So Japanese often builds the sentence first and then finishes with the action.

Also, particles like に, の, and を show how each part relates to the verb, so word order is more flexible than in English.

It is the polite non-past form, which can mean either present or future depending on context.

In this sentence, because of 今日中に (by the end of today), it is naturally understood as future:

• I will reply by today

Japanese does not have a separate future tense like English. The non-past form often covers both meanings.

Yes, it is polite because it ends with します.

Compare:

• 返事をします = polite
• 返事をする = plain dictionary form
• 返事をします is appropriate in normal polite conversation

The vocabulary itself is not extremely formal, but the verb ending makes the sentence polite.

No. Standard Japanese is normally written without spaces.

So this sentence would usually be written as:

今日中に友達のメールに返事をします。

Spaces are often added in teaching materials to make the parts easier to see, especially for beginners.