Du kan ringe til mig når som helst, hvis du har flere spørgsmål.

Because the Danish verb at ringe normally takes the preposition til when you mean to call someone.

• ringe til nogen = call someone
• ringe by itself = ring, phone, or give a call

So:

• Du kan ringe til mig = You can call me

This is different from English, where call usually takes a direct object without a preposition.

Because kan is a modal verb, and in Danish modal verbs are followed by the infinitive without at.

So:

• du kan ringe = you can call
• not du kan at ringe

This is the same pattern you get with other modal verbs:

• du vil komme = you want to come
• du skal gå = you must go
• du må spørge = you may ask

A good rule: after Danish modal verbs like kan, vil, skal, må, bør, you normally do not use at.

Here kan most naturally means something like can / are free to / may.

In context, Du kan ringe til mig når som helst is not really about physical ability. It is more like:

• You can call me anytime
• Feel free to call me anytime

So it often has a friendly, permissive meaning in sentences like this.

Når som helst is a fixed expression meaning any time, whenever, or at any time at all.

Word by word, it is not something you should translate too literally. The important thing is to learn it as a chunk:

• når som helst = any time / whenever

Examples:

• Du må komme når som helst. = You may come anytime.
• Ring når som helst. = Call anytime.

So in your sentence:

• Du kan ringe til mig når som helst = You can call me anytime

Yes, når often means when, but in når som helst it is part of a set phrase.

Compare:

• Når du kommer, spiser vi. = When you come, we eat.
• når som helst = any time / whenever

So når still has a time meaning, but the whole expression når som helst works idiomatically.

Because hvis du har flere spørgsmål is a subordinate clause introduced by hvis = if.

So the sentence is divided like this:

• Du kan ringe til mig når som helst = main clause
• hvis du har flere spørgsmål = subordinate clause

Many Danish sentences place a comma before a subordinate clause like this.

You may also notice that Danish comma usage has had more than one accepted system, so learners sometimes see variation. But a comma before hvis in a sentence like this is very common and perfectly normal.

Because spørgsmål is a countable noun: you can have one question, two questions, more questions.

For countable nouns, Danish usually uses flere:

• flere spørgsmål = more questions
• flere bøger = more books

Mere is more often used with uncountable things:

• mere tid = more time
• mere vand = more water

So flere spørgsmål is the natural choice.

Because Danish, like English, often uses no article with an indefinite plural noun after more.

Compare:

• more questions
• flere spørgsmål

You do not need an equivalent of some/the here. It is just an indefinite plural phrase.

If you wanted the questions, then you would need a definite form:

• spørgsmålene = the questions

But that is not the meaning here.

Because some Danish nouns have the same form in the indefinite singular and indefinite plural.

For et spørgsmål:

• singular indefinite: et spørgsmål = a question
• plural indefinite: spørgsmål = questions
• plural definite: spørgsmålene = the questions

In your sentence, we know it is plural because of flere:

• flere spørgsmål = more questions

So even though the noun itself does not change, the meaning is clearly plural.

Yes. Danish main clauses follow the verb-second rule, so if you move the subordinate clause to the front, the finite verb in the main clause comes before the subject.

Original order:

• Du kan ringe til mig når som helst, hvis du har flere spørgsmål.

With the if-clause first:

• Hvis du har flere spørgsmål, kan du ringe til mig når som helst.

Notice the change:

• du kan in the original main clause
• kan du after the fronted clause

This inversion is very important in Danish.

Not really. In normal speech, mig is usually pronounced something close to my, though the exact Danish sound is different from English.

This is common in Danish: spelling and pronunciation do not always match closely.

A few words in the sentence that learners often find tricky are:

• mig
• når
• spørgsmål
• helst

So if pronunciation feels difficult here, that is completely normal.

Yes, and it would be very similar, but the nuance changes a little.

• Du kan ringe til mig når som helst = You can call me anytime / feel free to call me anytime
• Du må ringe til mig når som helst = You may call me anytime

Kan often sounds very natural and friendly in everyday Danish for this kind of offer. Må can sound a bit more like explicit permission.

So the original sentence is a very idiomatic choice.

It is neutral and friendly, and it uses du, so it is the normal informal second-person singular form.

That makes it suitable for most everyday situations:

• speaking to a friend
• speaking to a classmate
• speaking to a colleague in a normal modern Danish context

Modern Danish uses du very widely, much more than learners sometimes expect.