Passagererne skulle gå om bord én ad gangen, fordi der kun var én dør.

Because -ne marks the definite plural in Danish.

Here are the forms:

• en passager = a passenger
• passagerer = passengers
• passagererne = the passengers

So passagererne is not just plural; it is specifically the passengers.

Here skulle is the past tense of skal.

In this sentence, it most naturally means something like:

• had to
• were supposed to
• were to

So it expresses a requirement or instruction in the past. It does not usually match English should very closely here.

Because skulle is a modal verb, and modal verbs are followed by an infinitive.

So:

• skulle = had to / were to
• gå = go

Together, skulle gå means had to go / were to go.

After modal verbs in Danish, you normally use the infinitive without at:

• jeg skal gå = I must go
• de kunne se det = they could see it
• vi ville spise = we wanted to eat

So skulle gå is completely normal.

Gå om bord is a fixed expression meaning to board or to go on board.

It is used when getting onto something like:

• a ship
• a ferry
• a plane

Literally, it is close to go on board, which is why English speakers can often recognize the logic of it.

The accent in én shows that it is the number one and that it is being stressed.

That helps distinguish it from en, which is also the common-gender indefinite article a/an.

So:

• en dør = a door
• én dør = one door

In this sentence, the number matters, so én is written with an accent for clarity and emphasis.

Ad gangen is part of a common Danish expression meaning at a time.

So:

• én ad gangen = one at a time
• to ad gangen = two at a time

You should learn én ad gangen as a set phrase. English speakers often try to analyze it word by word, but it is best understood as an idiomatic chunk.

Here der is an existential dummy subject, like English there in:

• There was only one door.

So der does not mean a physical location here. It is not there in the sense of over there.

Compare:

• Der var én dør. = There was one door.
• Bogen ligger der. = The book is there.

Those are two different uses of der.

Because fordi introduces a subordinate clause, and subordinate clauses in Danish usually have different word order from main clauses.

In a main clause, you often get:

• Der var kun én dør.

But after fordi, standard written Danish uses subordinate word order, where words like kun, ikke, and også usually come before the finite verb:

• fordi der kun var én dør

So the position of kun is a grammar clue that this is a subordinate clause.

Because the sentence describes a situation in the past.

• skulle = past tense
• var = past tense

The speaker is looking back on what the passengers had to do and what the situation was.

If you wanted to say it as a present rule, you could say:

• Passagererne skal gå om bord én ad gangen, fordi der kun er én dør.

That would mean the same kind of thing, but in the present.

Yes, but the nuance changes a little.

• skulle often sounds like an instruction, arrangement, or stated requirement
• måtte often sounds more like necessity forced by circumstances

So:

• Passagererne skulle gå om bord én ad gangen = the passengers were supposed/had to board one at a time
• Passagererne måtte gå om bord én ad gangen = the passengers had to board one at a time, with slightly more emphasis on necessity

In this sentence, skulle fits well because it sounds like a boarding procedure or rule.