Hvis en besked ligger i spam, ser min søster den ofte for sent.

Hvis means if and introduces a condition: if a message ends up in spam, then...

Danish når usually means when for something seen as expected or regular. Hvis is better here because the sentence is talking about a situation that may happen: a message being in spam.

So:

• Hvis en besked ligger i spam... = If a message is in spam...
• Når en besked ligger i spam... would sound more like When a message is in spam..., focusing more on a recurring/expected situation.

Both can be possible in some contexts, but hvis is the straightforward choice for a condition.

Because besked is a common-gender noun in Danish, so it takes en in the indefinite singular.

• en besked = a message
• beskeden = the message

This also matters later in the sentence, because the pronoun referring back to besked is den.

Literally, ligger means lies/is lying. But Danish often uses ligge for something that is located somewhere, especially when talking about emails, files, papers, messages, and similar things.

So en besked ligger i spam means:

• a message is in the spam folder
• more literally: a message is sitting/lying in spam

This is very natural Danish. English usually says is in spam, while Danish often prefers ligger i spam in this kind of context.

Because spam here is being used like a category or folder name, similar to English in spam or in junk mail.

So i spam is natural Danish for:

• in spam
• in the spam folder

If you want to be more explicit, you could say something like i spammappen = in the spam folder, but i spam is very common and idiomatic.

This is because Danish main clauses normally follow the V2 rule: the finite verb comes in the second position.

Here, the sentence starts with a subordinate clause:

• Hvis en besked ligger i spam

After that whole clause, the main clause begins. Since the first position has already been taken by the subordinate clause, the finite verb of the main clause must come next:

• ser min søster den ofte for sent

So the structure is:

1. Hvis en besked ligger i spam = first position
2. ser = finite verb in second position
3. min søster = subject

If you wrote Hvis en besked ligger i spam, min søster ser den..., that would be wrong in standard Danish.

Den refers back to en besked.

Since besked is a singular common-gender noun, the matching pronoun is den:

• en besked → den

So:

• ser min søster den = my sister sees it

If the noun were a singular neuter noun, you would use det instead.

In Danish, short object pronouns such as den, det, ham, hende often come quite early in the clause, before many adverbs.

So this order is natural:

• ser min søster den ofte for sent

Here the order is:

• ser = verb
• min søster = subject
• den = object pronoun
• ofte = adverb of frequency
• for sent = final adverbial phrase

That placement sounds normal and idiomatic in Danish.

Ofte means often. In this sentence it comes after the object pronoun den and before for sent.

So:

• ser min søster den ofte for sent = my sister often sees it too late

This is a very natural position for ofte in Danish. Frequency adverbs often come in the middle part of the clause, but short pronouns like den commonly come before them.

For sent means too late, not just late.

• sent = late
• for sent = too late

So the idea is not only that she sees it late, but that she sees it later than is useful or appropriate.

Ser literally means sees, but in Danish it can also mean notices, checks, or comes to see a message.

So ser min søster den ofte for sent means something like:

• my sister often notices it too late
• my sister often sees it too late

If you said læser, that would focus more specifically on reading the message itself:

• læser min søster den ofte for sent = my sister often reads it too late

Both can be possible, but ser is very natural if the main point is that she does not notice the message in time.

Danish often uses the present tense for general truths, habits, and repeated situations, just like English does.

So this sentence describes a typical pattern:

• Hvis en besked ligger i spam, ser min søster den ofte for sent.

It means this is what usually happens, not just what is happening right now.

Because Hvis en besked ligger i spam is a subordinate clause, and it is followed by the main clause.

So the comma marks the boundary between:

• Hvis en besked ligger i spam
  and
• ser min søster den ofte for sent

This is normal Danish punctuation.