fairu no appuroodo ga owattara, sensei ni meeru wo okurimasu

の links two nouns together.

So ファイルのアップロード means the upload of the file or more naturally the file upload.

A useful way to think about it is:

• ファイル = file
• アップロード = upload
• ファイルのアップロード = file upload / uploading the file

Japanese often uses A の B where English might use a compound noun like file upload.

Because 終わる is an intransitive verb.

• 終わる = to end / to finish
• 終える = to finish something

With 終わる, the thing that ends is marked by が, not を.

So:

• アップロードが終わる = the upload finishes
• アップロードを終える = to finish the upload

In this sentence, the focus is on the upload being finished, so が終わったら is natural.

終わったら is the たら-form of 終わる.

Breakdown:

• 終わる = to finish
• 終わった = finished
• 終わったら = if/when it has finished

So アップロードが終わったら means:

• when the upload finishes
• after the upload is finished
• sometimes if the upload finishes, depending on context

Here, when/after is the most natural interpretation.

This is very common in Japanese grammar.

In the たら conditional, Japanese uses the past short form before ら, but that does not necessarily mean past time in English.

So:

• 終わったら does not mean if it finished
• it means if/when it finishes or after it has finished

It is a grammar pattern, not a literal past-time meaning.

It can mean either, but in this sentence it is most naturally when or after.

Why? Because uploading a file is something expected to happen, so the sentence is describing the next action in sequence:

• first: the upload finishes
• next: I send an email to the teacher

So here たら is closer to:

• when the upload is finished
• once the upload finishes
• after the upload finishes

Because Japanese normally places the main verb at the end of the clause or sentence.

This sentence has two parts:

1. ファイルのアップロードが終わったら
   = when the file upload is finished

2. 先生にメールを送ります
   = I will send an email to the teacher

The final main verb is 送ります, so it comes at the end.

That is normal Japanese word order.

The comma marks the boundary between the conditional/time clause and the main clause.

So it separates:

• when the file upload is finished from
• I will send an email to the teacher

Japanese commas are often more flexible than English commas, but here the comma makes the sentence easier to read.

You may also sometimes see similar sentences without a comma, especially in casual writing.

Because 先生 is the recipient of the email, not the direct object.

• に marks the target/recipient
• を marks the direct object

In 先生にメールを送ります:

• 先生に = to the teacher
• メールを = email
• 送ります = send

So the thing being sent is the email, and the person receiving it is the teacher.

Usually に is the more natural choice here.

Both に and へ can sometimes mark direction or destination, but:

• に strongly marks the recipient/target
• へ emphasizes direction more than recipient

With 送る, both may appear in some contexts, but 先生にメールを送ります is the standard, natural phrasing for send an email to the teacher.

Because メールを送る is the standard expression meaning to send an email.

• メール = email
• 送る = to send

So メールを送ります literally means send an email.

You may hear メールする in casual Japanese, meaning to email, but メールを送ります is clearer and very natural, especially in polite or careful speech.

Because in Japanese, subordinate clauses usually use the plain form, even when the main sentence is polite.

So this is normal:

• 終わったら = plain form in the clause
• 送ります = polite main verb

Japanese does not usually make every verb polite inside one sentence. The politeness is normally shown in the main clause.

So this sentence is grammatically and socially natural.

The subject is omitted, which is very common in Japanese.

From context, 送ります usually implies I will send here.

So the full meaning is understood as something like:

• When the file upload is finished, I will send an email to the teacher.

Japanese often leaves out subjects like I, you, or they when they are obvious from context.

They are related, but not exactly the same in feel.

• ファイルのアップロード = the upload of the file / file upload
  
  • noun phrase
  • compact and technical-sounding
• ファイルをアップロードすること = uploading the file / the act of uploading the file
  
  • more verbal/explanatory

In your sentence, ファイルのアップロード sounds very natural, especially in computer-related situations.

No, in normal Japanese writing, spaces are usually not used like this.

The sentence would normally be written as:

ファイルのアップロードが終わったら、先生にメールを送ります。

Spaces are sometimes added in teaching materials to help beginners see the parts more clearly, but they are not standard in ordinary Japanese text.

Not literally in its current form.

The sentence as written is:

ファイルのアップロードが終わったら = when the file upload is finished

This focuses on the upload finishing, not explicitly on you finishing it.

If you wanted to emphasize I finish uploading the file, you might say something like:

ファイルをアップロードし終えたら、先生にメールを送ります。

But in many everyday contexts, the original sentence is completely natural and may imply your own upload anyway. The original is just less personal and more event-focused.