Я переслал файл начальнице ещё раз, потому что она не открыла вложение.

Переслал comes from переслать, which means to send/forward again or to resend.

In this sentence, ещё раз also shows that the file was sent again, so переслал is very natural.

Comparison:

• послал = sent
• отправил = sent/dispatched
• переслал = resent / forwarded again

So Я переслал файл... means something like I resent the file...

Because начальнице is in the dative case, which is used for the recipient of something.

The basic form is:

• начальница = female boss / manager

Dative singular:

• начальнице = to the boss

So:

• Я переслал файл начальнице = I resent the file to my boss

This is very common with verbs of giving, sending, showing, telling, etc.

Yes. Начальница is specifically a female boss.

Compare:

• начальник = male boss / manager
• начальница = female boss / manager

So this sentence tells us that the recipient is a woman.

Here файл is in the accusative case, because it is the direct object of переслал.

For many masculine inanimate nouns, the accusative singular looks the same as the nominative singular:

• nominative: файл
• accusative: файл

So:

• Я переслал файл = I resent the file

Ещё раз means again, one more time, or once more.

Literally:

• ещё = still / more
• раз = time / occasion

Together:

• ещё раз = one more time / again

So:

• Я переслал файл начальнице ещё раз = I resent the file to my boss again

It can feel a little repetitive in English, but in Russian this is completely normal.

Both elements reinforce the idea:

• переслал = resent
• ещё раз = again / one more time

Using both can make the sentence sound more explicit and natural, especially in everyday speech.

Потому что means because.

It introduces the reason:

• Я переслал файл начальнице ещё раз, потому что...
• I resent the file to my boss again, because...

This is one of the most common ways to say because in Russian.

Не открыла is the perfective past, from открыть, and it usually means did not open as a completed event.

Here the idea is that she failed to open the attachment on that occasion, so the perfective form fits well:

• она не открыла вложение = she did not open the attachment

If you said не открывала, that would usually suggest a more process-oriented or repeated idea, such as she wasn’t opening it / she never opened it over some period, depending on context.

Because the subject is она.

In the past tense, Russian verbs agree with the subject in gender and number:

• он открыл = he opened
• она открыла = she opened
• оно открыло = it opened
• они открыли = they opened

So:

• она не открыла = she did not open

Вложение means attachment, especially in email or messaging contexts.

So:

• открыть вложение = to open the attachment

In this sentence, it most likely refers to the attached file in an email or message.

Because вложение is the direct object of открыла, so it is in the accusative singular.

For neuter inanimate nouns, accusative singular is the same as nominative singular:

• nominative: вложение
• accusative: вложение

So:

• она не открыла вложение = she didn’t open the attachment

Yes, in real context it can strongly suggest that meaning.

Literally, не открыла means did not open, but depending on the situation it may imply:

• she didn’t manage to open it
• she couldn’t open it
• she failed to open it

Russian often leaves that distinction to context.

Yes, Russian word order is relatively flexible.

Neutral order here:

• Я переслал файл начальнице ещё раз, потому что она не открыла вложение.

This sounds natural and straightforward.

You could also move things for emphasis, for example:

• Я ещё раз переслал файл начальнице...
  Slightly emphasizes again
• Начальнице я переслал файл ещё раз...
  Emphasizes to my boss
• Файл я переслал начальнице ещё раз...
  Emphasizes the file

But the original version is a good standard sentence.

Not always. Russian often drops subject pronouns when the meaning is clear from the verb form.

So you could say:

• Переслал файл начальнице ещё раз, потому что она не открыла вложение.

That can still mean I resent the file..., especially in context.

But Я is included when the speaker wants to be explicit, contrastive, or just clear.

The main clause is:

• Я переслал файл начальнице ещё раз
  = I resent the file to my boss again

The subordinate reason clause is:

• потому что она не открыла вложение
  = because she didn’t open the attachment

A useful way to see it:

• action: I resent the file
• reason: because she didn’t open the attachment

In most contexts here, I resent the file is the best translation.

Why?

• ещё раз clearly means again
• потому что она не открыла вложение suggests the first sending did not work

Although переслать can also mean forward, this sentence strongly points to resend.

Yes. Russian often omits possessives when they are obvious from context.

So начальнице literally means to the female boss / manager, but in English the most natural translation is often:

• to my boss
• to my manager

The sentence does not explicitly say my, but that is often understood.