Vil De forresten sige til, hvis De overhovedet ikke kan åbne filen?

De is the formal way to say you in Danish, and it is traditionally capitalized.

So in this sentence, De means you, but in a polite, respectful, somewhat formal way.

A few useful points:

• De = formal singular or plural you
• du = normal informal singular you
• I = informal plural you (less common in many everyday situations, where people often still use I or just restructure the sentence)

In modern Danish, De is much less common than it used to be. Most people would normally say du unless they want to sound especially formal, old-fashioned, or very polite.

A more everyday version would be:

• Vil du forresten sige til, hvis du overhovedet ikke kan åbne filen?

Here vil does not mainly express future time. Instead, it helps form a polite request.

So Vil De sige til ... ? is similar to:

• Would you let me know ... ?
• Could you let me know ... ?

Literally, vil is related to want to / will, but in questions it often works like a polite English request.

Compare:

• De vil åbne filen. = You want to open the file / You will open the file (depending on context)
• Vil De åbne filen? = Will/Would you open the file?

So in this sentence, Vil De sige til ... ? is best understood as a polite request, not a plain future statement.

Forresten means something like:

• by the way
• incidentally

It adds a side remark or extra point that the speaker wants to mention.

In this sentence:

• Vil De forresten sige til ... ?
  = Would you, by the way, let me know ... ?

It makes the request sound a bit more conversational, as if the speaker is adding one more thing.

Sige til is a common Danish expression meaning:

• let someone know
• tell someone
• say so
• inform someone

So sige til is not just the basic verb sige (to say). The combination sige til has its own idiomatic meaning.

Examples:

• Sig til, hvis du har spørgsmål.
  = Let me know if you have questions.

• Du skal sige til, når du er klar.
  = You should let me know when you're ready.

In your sentence, the object is not stated explicitly. Danish often leaves it out when it is obvious from context. In fuller form, it could be:

• Vil De sige til mig, hvis De ... ?
  = Would you let me know, if you ... ?

Because hvis (if) introduces a subordinate clause, and Danish subordinate clauses usually have a different word order from main clauses.

Main clause word order often has the finite verb in second position:

• De kan ikke åbne filen.
  = You cannot open the file.

But in a subordinate clause, adverbs like ikke usually come before the finite verb:

• ..., hvis De ikke kan åbne filen.
  = ..., if you cannot open the file.

That is why you get:

• hvis De overhovedet ikke kan åbne filen

Structure:

• hvis = if
• De = you
• overhovedet ikke = at all not / not at all
• kan = can
• åbne filen = open the file

So the placement of ikke before kan is normal in a subordinate clause.

Overhovedet ikke means:

• not at all
• absolutely not
• cannot ... at all

It strengthens the negation.

So:

• ikke kan åbne filen = cannot open the file
• overhovedet ikke kan åbne filen = cannot open the file at all

It suggests complete inability, not just difficulty.

For example:

• Jeg forstår det ikke. = I don't understand it.
• Jeg forstår det overhovedet ikke. = I don't understand it at all.

In your sentence, it makes the request more specific: the speaker wants to be told if the recipient cannot open the file in any way whatsoever.

Filen is the definite form of fil:

• en fil = a file
• filen = the file

Danish usually adds the definite article to the end of the noun:

• en bog = a book
• bogen = the book

Here, filen is used because both speaker and listener know which file is being talked about.

So:

• åbne filen = open the file

Because hvis De overhovedet ikke kan åbne filen is a subordinate clause, and Danish punctuation normally separates such clauses with a comma.

So the comma marks the boundary between:

• main clause: Vil De forresten sige til
• subordinate clause: hvis De overhovedet ikke kan åbne filen

This is very standard in written Danish.

Yes, absolutely. In modern Danish, that would usually be more natural in everyday life.

Informal version:

• Vil du forresten sige til, hvis du overhovedet ikke kan åbne filen?

That sounds much more normal in most modern situations.

The version with De sounds:

• formal
• polite
• somewhat old-fashioned
• possibly customer-service-like or written in a very respectful tone

So both are grammatically fine, but they give different social tones.

It is polite.

Several things make it polite:

• Vil in a question softens the request
• De is formal and respectful
• the sentence is phrased as a request for information, not a command

So it sounds more polite than something like:

• Sig til, hvis De ikke kan åbne filen.
  = Let me know if you can't open the file.

That version is still fine, but more direct.

Your sentence is closer to:

• Would you please let me know if you can't open the file at all?

Not usually. Although sige means say and til often means to, the combination sige til is idiomatic.

A literal translation say to would sound wrong or incomplete in English here.

Better translations are:

• let me know
• tell me
• inform me
• say so

So in this sentence, it is better to think of sige til as one expression rather than translating each word separately.

Yes, mainly De.

The rest of the sentence is perfectly normal Danish:

• vil
• forresten
• sige til
• hvis
• overhovedet ikke
• kan åbne filen

All of that is standard and natural.

What makes it sound formal is mostly the use of De instead of du. Without that, the sentence would feel very ordinary and modern.