tomodati kara no meeru ni mada hensinsite imasen.

から means from. In this sentence, 友達から means from a friend.

To connect that idea to the noun メール, Japanese uses の:

• 友達からのメール = an email from a friend

So からの is not one single particle; it is:

• から = from
• の = links the phrase to the noun that follows

A learner often compares this with 友達のメール. That can also mean a friend's email, but 友達からのメール is clearer when you specifically want to say the email was sent by the friend.

Because 返信する commonly uses に to mark the thing or person you are replying to.

• メールに返信する = reply to the email
• 友達に返信する = reply to a friend

So in this sentence:

• メールに = to the email

This is similar to English reply to.

You may also see related patterns such as:

• メールの返信をする = do/write a reply to the email

But in the sentence you gave, に is the natural particle with 返信する.

With a negative verb, まだ usually means not yet.

So:

• まだ 返信していません = have not replied yet

This is a very common pattern:

• まだ食べていません = I haven’t eaten yet
• まだ行っていません = I haven’t gone yet

Be careful: with an affirmative verb, まだ often means still instead.

• まだ食べています = I am still eating

So the meaning of まだ depends a lot on whether the sentence is positive or negative.

This is a very common learner question.

• 返信しません often sounds like I will not reply or I do not reply
• 返信していません means I have not replied / I am not in the state of having replied

With まだ, Japanese strongly prefers 〜ていません for not yet:

• まだ返信しません sounds unnatural for haven’t replied yet
• まだ返信していません is natural

So 返信していません focuses on the current situation: as of now, the reply has not been done.

返信 is a suru-verb noun.

That means:

• 返信 = reply / response
• 返信する = to reply

In the sentence:

• 返信していません

this comes from:

• 返信する → 返信している → 返信していません

So even though 返信 looks like a noun by itself, it can become a verb when used with する.

Japanese often leaves out the subject when it is clear from context.

So although the sentence does not say I, the natural understanding is usually:

• I haven’t replied yet to the email from my friend

Japanese does this all the time. If the speaker, listener, or topic is already understood, it is usually omitted.

A fully explicit version might be something like:

• 私は友達からのメールにまだ返信していません。

But in normal conversation, that 私は is often unnecessary.

Yes. It is in polite style because it ends with いません.

• 返信していません = polite
• 返信していない = plain

So the plain-style version would be:

• 友達からのメールにまだ返信していない。

The polite version is appropriate in many everyday situations, especially when speaking carefully or formally.

No. Standard Japanese is normally written without spaces.

So the sentence would usually appear as:

• 友達からのメールにまだ返信していません。

Spaces are often added in textbooks or beginner materials to make the structure easier to see.

Yes, to some extent. Japanese word order is more flexible than English, as long as the particles stay attached to the right words.

For example, these are both natural:

• 友達からのメールにまだ返信していません。
• まだ友達からのメールに返信していません。

Both mean basically the same thing: I haven’t replied yet to the email from my friend.

However, the version with まだ earlier may put a little more focus on the not yet part.