sensei kara meeru ga kitara, sugu ni hensinsimasu.

から marks the source or starting point of something. Here, it means the email is from the teacher.

So:

• 先生から = from the teacher

This is different from 先生のメール, which can also mean the teacher’s email, but that expression can sound more like an email belonging to the teacher or an email written by the teacher as a noun phrase. In this sentence, 先生からメールが来た focuses on the event of an email coming from the teacher.

Because メール is the thing that comes, so it is the subject of 来る.

• メールが来る = an email comes / arrives
• not メールを来る because 来る does not take a direct object here

In English, we often think I receive an email, but Japanese often phrases this as an email comes. That is why メール gets が, not を.

来たら is the -tara conditional, and it often means if/when it comes.

In this sentence, it works very naturally because the second action happens after the first one is completed:

1. the email arrives
2. then I reply

So 来たら gives the sense of:

• when an email comes
• if an email comes

Compared with other forms:

• 来ると often sounds more like a natural consequence or repeated result
• 来れば is also if it comes, but can sound a bit more conditional or formal depending on context
• 来たら is very common for everyday situations where one action follows another

This is a very common learner question. In Japanese, the -たら conditional is built from the past plain form + ら, but it does not necessarily mean past time.

So:

• 来た = came
• 来たら = if/when it comes/came

The past-looking form is just part of the grammar pattern here. It signals that the first action is treated as completed before the next action happens.

So the nuance is:

• after the email has arrived, I reply right away

すぐに means immediately / right away.

The に is often used with adverbial expressions, and in this case it makes すぐ function clearly as an adverb.

Both of these are common:

• すぐ返信します
• すぐに返信します

The version with に can sound a little more complete or slightly more formal, but in many cases the difference is small.

返信 is a noun meaning reply / response, especially in contexts like email, text, or messages.
返信します is a suru-verb pattern:

• 返信する = to reply
• 返信します = polite form

So this is a very common structure in Japanese:

• 勉強する = to study
• 電話する = to call
• 返信する = to reply

In this sentence, 返信します is a natural choice because the topic is email.

Both relate to replying, but they are used a little differently.

• 返事 = reply, answer, response in a broad sense
  
  • can be spoken or written
  • often used for answering someone directly
• 返信 = reply sent back, especially for messages, email, texts, etc.

So with メール, 返信 is especially natural:

• メールに返信する = reply to an email

You could talk about 返事をする, but 返信する matches the email context more directly.

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

So 返信します does not explicitly say:

• I reply
• we reply
• he/she replies

But because the sentence ends in the polite form します, and because this kind of sentence is often said by the speaker about their own actions, the natural interpretation is usually:

• I will reply
• I reply

This omission is completely normal in Japanese.

A sentence does not always need は. Japanese often omits the topic if it is already clear.

Here, the sentence can stand naturally without it:

• 先生からメールが来たら、すぐに返信します。

If you wanted to add a topic, you could say:

• 私は、先生からメールが来たら、すぐに返信します。

That would explicitly mark I as the topic, but in many real conversations it is unnecessary.

Also, メールが is important here because メール is the subject of 来る.

It can mean either, depending on context. In this sentence, it often feels like when if the speaker expects the email to come, or if if it is only a possibility.

So the Japanese form is flexible:

• expected situation: when the teacher emails me
• possible situation: if the teacher emails me

This is normal with -たら. English forces you to choose more clearly, but Japanese often leaves it open.

It can do either.

With a sentence like this, the non-past form 返信します can mean:

• a habitual action: Whenever the teacher emails me, I reply right away
• a future intention: If/When the teacher emails me, I’ll reply right away

Japanese often lets context decide which meaning is intended.

Yes, but Japanese also uses 来る for things like messages, calls, and emails arriving.

So:

• メールが来る = an email comes / arrives
• 電話が来る = a call comes
• 連絡が来る = contact/message comes

This may feel different from English, where we often say receive an email, but it is very natural Japanese.

Yes, but the structure would be different.

• 先生からメールが来たら = if/when an email comes from the teacher
  
  • focuses on the event
• 先生からのメール = an email from the teacher
  
  • this is a noun phrase

For example:

• 先生からのメールをもらったら、すぐに返信します。
• 先生からのメールが来たら、すぐに返信します。

In your original sentence, the speaker uses the event pattern メールが来る, which is very natural.

Not completely. Japanese word order is more flexible than English, as long as the particles make the roles clear.

The original sentence:

• 先生からメールが来たら、すぐに返信します。

is the most natural and straightforward order.

You might also hear small variations like:

• 先生からメールが来たら、返信はすぐにします。

But the original is simpler and more neutral.

The main thing is that the conditional clause 先生からメールが来たら comes before the main action すぐに返信します, which is very common and easy to understand.

します is the non-past polite form. In Japanese, the non-past form can refer to:

• present habits
• future actions

So here it can mean:

• I reply right away as a habit
• I will reply right away in the future

If you used しました, that would make the reply a completed past action, which would not fit this sentence.

So 返信します is the correct form for a general or future statement.