Jeg ville ikke fortælle den hemmelighed til hvem som helst.

Ville is formally the past tense of vil, but in Danish it is also often used like English would.

So here Jeg ville ikke fortælle ... usually means something like:

• I wouldn’t tell ...
• I would not tell ...

It does not have to refer to the past. It can express:

• a hypothetical attitude
• a general principle
• a softer, less direct tone

Compare:

• Jeg vil ikke fortælle den hemmelighed. = I won’t / don’t want to tell that secret.
  More direct.
• Jeg ville ikke fortælle den hemmelighed. = I wouldn’t tell that secret.
  More conditional or less blunt.

Because ville is a modal verb, and modal verbs in Danish normally take the bare infinitive.

So you say:

• jeg vil fortælle
• jeg ville fortælle
• jeg kan fortælle
• jeg skal fortælle

Not:

• jeg ville at fortælle

This works much like English would tell, can tell, must tell.

Because ville is the finite verb in the sentence, and in a main clause Danish usually places ikke after the finite verb.

So the pattern is:

• Jeg ville ikke fortælle ...

Compare:

• Jeg fortæller ikke hemmeligheden.
• Jeg kan ikke komme.
• Jeg vil ikke gå.

A useful rule of thumb is:

• in a main clause, ikke usually comes after the finite verb
• in a subordinate clause, it usually comes before the finite verb

For example:

• Jeg ved, at jeg ikke ville fortælle den hemmelighed til hvem som helst.

This is a very natural question, because both forms can relate to the secret, but they are not exactly the same.

• hemmeligheden = the secret
  the normal definite form
• den hemmelighed = more like that secret or that particular secret

In Danish, if you use a separate determiner like den, the noun itself stays in its basic form:

• den bog
• den idé
• den hemmelighed

So den hemmelighed is not built with the ending -en, because den is already doing that job.

Nuance:

• hemmeligheden is often the more neutral way to say the secret
• den hemmelighed often points to a specific secret already known in the context, with a slightly demonstrative feel

Yes. Fortælle can be used in two common patterns:

1. fortælle nogen noget
   = tell someone something

2. fortælle noget til nogen
   = tell something to someone

So both of these are possible patterns in Danish:

• Hun fortalte mig sandheden.
• Hun fortalte sandheden til mig.

Your sentence uses the second pattern:

• fortælle den hemmelighed til hvem som helst

Using til is especially clear and natural when the receiver is a longer phrase, such as hvem som helst.

Hvem som helst is a fixed expression meaning:

• anyone at all
• just anyone
• whoever

In this sentence, it means the speaker would not tell the secret to just any person.

So the phrase adds emphasis. It is stronger than a plain nogen in many contexts.

Compare:

• Jeg ville ikke fortælle den hemmelighed til nogen.
  = I wouldn’t tell that secret to anyone.
• Jeg ville ikke fortælle den hemmelighed til hvem som helst.
  = I wouldn’t tell that secret to just anyone.

The second one has more of the idea that the secret would only be shared with the right person.

By itself, yes, hvem means who/whom. But hvem som helst is a set phrase, and together it means anyone at all.

So in this sentence, hvem is not functioning as a question word. The whole phrase acts as an indefinite expression.

Danish has several similar fixed patterns:

• hvem som helst = anyone
• hvad som helst = anything
• hvor som helst = anywhere
• når som helst = anytime

So it is best to learn hvem som helst as one chunk.

Yes. That would be perfectly grammatical and very common:

• Jeg ville ikke fortælle den hemmelighed til nogen.

This is simpler and very natural.

The difference is mostly nuance:

• til nogen = to anyone
  neutral
• til hvem som helst = to just anyone / to absolutely anybody
  more emphatic

If you want the idea of not to just anybody, the original phrasing is especially good.

Not necessarily.

Even though ville is historically a past-tense form, here it often expresses a conditional or general hypothetical meaning, not past time.

So the sentence can mean something like:

• I wouldn’t tell that secret to just anyone.

If you really wanted to make it clearly about the past, context would usually show that:

• Dengang ville jeg ikke fortælle den hemmelighed til hvem som helst.
  = Back then, I wouldn’t / didn’t want to tell that secret to just anyone.

Without that kind of context, learners should usually read ville here as would, not as simple past time.

Yes, somewhat. Danish often moves sentence parts to the front for emphasis, but the finite verb still stays in second position in a main clause.

Basic order:

• Jeg ville ikke fortælle den hemmelighed til hvem som helst.

With emphasis on the object:

• Den hemmelighed ville jeg ikke fortælle til hvem som helst.

This is a common Danish feature called V2 word order: the finite verb comes second in main clauses.

So even if you move Den hemmelighed to the front, ville still has to come right after it.