Den ven, hvis konto blev lukket i går, måtte ringe til banken med det samme.

Yes, it is the same Danish word, but here it does not mean if. In this sentence, hvis means whose.

So:

• den ven, hvis konto... = the friend whose account...

Danish uses hvis both for:

• if in some contexts
• whose in relative clauses

The structure here clearly shows it is the whose meaning, because it introduces extra information about ven.

Because after a possessive word in Danish, the noun is normally in the indefinite form.

So Danish says:

• min ven = my friend
• hans konto = his account
• hvis konto = whose account

Not:

• min vennen
• hans kontoen
• hvis kontoen

This is very similar to English: you say his account, not his the account.

Den ven refers to a specific friend, not just any friend.

• en ven = a friend
• den ven = that friend / the friend in question

Here the speaker has a particular friend in mind, and the relative clause hvis konto blev lukket i går identifies which friend.

So the beginning means something like:

• That/the friend whose account was closed yesterday...

Sometimes, yes, depending on style and context.

• vennen = the friend
• den ven = often that friend or the particular friend

In this sentence, den ven sounds like the speaker is pointing out or identifying a specific person by the following clause. It has a slightly more selective feel: that friend, the one whose account was closed yesterday.

So den ven is not just random; it helps single out one friend.

Blev lukket is a past passive form.

Breakdown:

• blev = past tense of blive (become / get / be in passive constructions)
• lukket = past participle of lukke (close)

Together:

• blev lukket = was closed

So:

• konto blev lukket i går = the account was closed yesterday

This is a very common Danish way to make the passive.

Good question. Both can often be translated with was closed, but they are not exactly the same.

• blev lukket focuses on the event/action of being closed
• var lukket often describes the state/result of already being closed

So here:

• konto blev lukket i går = the account got closed / was closed yesterday
  → focus on what happened yesterday

If you said:

• konto var lukket i går
  it could sound more like the account was in a closed state yesterday

Because the sentence is about something that happened, blev lukket is the natural choice.

Here måtte means had to.

It is the past form of må, which often means must / may depending on context. In this sentence, it clearly means obligation or necessity:

• måtte ringe til banken = had to call the bank

So it does not mean might here.

Because after a modal verb in Danish, the next verb usually appears in the bare infinitive, without at.

That is the same basic idea as in English:

• must call
• not must to call

Here:

• måtte ringe = had to call

Other similar Danish examples:

• kan komme = can come
• skal gå = must go / will go
• vil spise = want to / will eat depending on context

So måtte ringe is completely normal.

Because ringe normally takes til when you mean call someone / phone someone.

So Danish says:

• ringe til banken = call the bank
• ringe til sin mor = call one’s mother

Even though English often says just call the bank, Danish usually needs the preposition til with ringe in this meaning.

Because it refers to a specific bank—most naturally, the bank connected with that account.

• banken = the bank
• en bank = a bank

If your account is closed, you would normally call the bank that handles the account, not just any bank.

So the definite form makes sense here.

Med det samme is a fixed expression meaning:

• immediately
• right away
• at once

So:

• måtte ringe til banken med det samme = had to call the bank immediately

It is very common in everyday Danish.

A close synonym is straks.

Because that part is a relative clause, and Danish normally places a comma before such clauses.

So the sentence is divided like this:

• Den ven,
• hvis konto blev lukket i går,
• måtte ringe til banken med det samme.

The middle part gives extra identifying information about den ven.

This kind of punctuation is very normal in written Danish.

Yes. It is normal Danish subordinate-clause word order.

Inside that clause, you have:

• hvis = whose
• konto = subject
• blev = finite verb
• lukket = participle
• i går = time expression

So:

• hvis konto blev lukket i går = whose account was closed yesterday

Nothing unusual is happening there. It is a straightforward relative clause.

The sentence has three main parts:

1. Den ven
   = the main noun phrase

2. hvis konto blev lukket i går
   = a relative clause describing that friend

3. måtte ringe til banken med det samme
   = the main clause verb phrase

So the skeleton is:

• [The friend], [whose account was closed yesterday], [had to call the bank immediately].

This is a useful pattern to learn, because Danish often packs a lot of information into a noun phrase with a relative clause.