Børnene går ind én ad gangen, når læreren åbner døren.

In Danish, the definite article is usually added to the end of the noun instead of being a separate word.

• et barn = a child
• børn = children
• børnene = the children

So -ene here is the definite plural ending.

Also, this noun is irregular:

• singular: barn
• plural: børn

So you cannot just predict the plural from the singular; you have to learn it as a vocabulary item.

Danish often uses a verb + adverb/particle where English might use a single verb.

• gå ind = go in / enter
• går = present tense of gå (to go)
• ind = in / inside

So Børnene går ind literally means The children go in.

This is very natural in Danish. You could sometimes use a more formal single verb like entrer, but går ind is much more common in everyday language.

Én ad gangen means one at a time.

It is a very common Danish expression:

• én = one
• ad gangen = at a time / one by one in sequence

So the whole phrase means that they go in individually, not all together.

Although the literal parts may not seem obvious at first, it is best learned as a fixed expression:

• én ad gangen = one at a time

The accent in én helps show that it means the number one, not the indefinite article en (a/an).

Compare:

• en lærer = a teacher
• én lærer = one teacher

In practice, Danish does not always require the accent, but writers often use it when they want to make the meaning extra clear or emphasize the number.

So in én ad gangen, the accent highlights the idea of one at a time.

Again, Danish usually puts the definite article at the end of the noun.

• en lærer = a teacher
• læreren = the teacher
• en dør = a door
• døren = the door

So:

• læreren åbner døren = the teacher opens the door

This is one of the biggest structural differences from English. English uses a separate word (the), while Danish usually uses a suffix.

Because når læreren åbner døren is a subordinate clause, and Danish standard spelling normally puts a comma before subordinate clauses.

So the sentence is divided like this:

• Børnene går ind én ad gangen = main clause
• når læreren åbner døren = subordinate clause

That is why you write:

• Børnene går ind én ad gangen, når læreren åbner døren.

In English, comma use here is more flexible. In Danish, the comma is much more systematic.

This is a very common learner question.

• når is used for when in the sense of whenever, when(ever) this happens, or often for future situations
• da is usually used for when referring to one specific event in the past

In this sentence, the meaning is something like:

• The children go in one at a time whenever the teacher opens the door

That is a repeated or general situation, so når is the right choice.

If you were talking about one single past event, da might be used instead.

Both går and åbner are in the present tense.

• går = go / are going
• åbner = opens / is opening

Danish, like English, often uses the present tense for:

• general truths
• habits
• routines
• instructions or descriptions of what normally happens

So this sentence sounds like a general rule or routine:

• The children go in one at a time when the teacher opens the door

It is not necessarily happening right now; it can describe what usually happens.

Because når læreren åbner døren is a subordinate clause, and subordinate clauses do not use the normal main-clause inversion pattern.

In a subordinate clause, the usual order is:

• subject + verb

So:

• når læreren åbner døren = when the teacher opens the door

If this were a main clause, Danish often follows the verb-second rule, but subordinate clauses are different.

That is why:

• Når læreren åbner døren, går børnene ind én ad gangen.

is correct, not:

• Når åbner læreren døren ... unless you are asking a question like When does the teacher open the door?

Yes. If the subordinate clause comes first, the main clause follows the Danish verb-second rule.

So you can say:

• Børnene går ind én ad gangen, når læreren åbner døren.

or

• Når læreren åbner døren, går børnene ind én ad gangen.

Notice what happens in the second version:

• går comes before børnene

This is because the first position is already occupied by the whole subordinate clause:

• Når læreren åbner døren

Then the finite verb of the main clause must come second:

• går

So Danish word order changes depending on what comes first in the sentence.

These are two vowels that English speakers often find difficult.

• ø in børnene and døren has no exact English equivalent. It is similar to German ö or French eu in deux.
• å in går and når is roughly like the vowel in thought for many English speakers, though the exact sound depends on accent.

A rough guide:

• børnene sounds approximately like BUR-nuh-nuh, but with a rounded vowel that English does not really have
• går sounds a bit like gor with a broad rounded vowel
• når sounds similar to nor

These are only approximations. For pronunciation, listening to native audio is much more useful than English spelling.