Hvis ingen åbner døren, ringer jeg på igen.

Because Danish main clauses normally follow the verb-second rule, often called V2.

When the sentence begins with a different element—here, the if-clause Hvis ingen åbner døren—that whole clause takes the first position. In the main clause, the finite verb must then come next:

• Hvis ingen åbner døren, ringer jeg på igen.

So the order is:

• first: Hvis ingen åbner døren
• second: ringer
• then: jeg

If you said Hvis ingen åbner døren, jeg ringer på igen, it would sound ungrammatical in standard Danish.

Ringe på is a fixed expression meaning to ring the doorbell or to ring at someone’s door.

So:

• at ringe = to ring / to call
• at ringe på = to ring the bell at a door

The på is part of the expression. Danish often has verb + particle combinations like this.

Examples:

• Jeg ringer på. = I ring the doorbell.
• Hun ringede på døren. = She rang the doorbell / rang at the door.

Without på, ringe often means something else, such as to call:

• Jeg ringer til ham. = I call him.

In Danish, particles like på in expressions such as ringe på are often placed later in the clause, especially after the subject:

• ringer jeg på igen

This is normal Danish word order.

You can think of ringe på as one unit in meaning, even though the words may be separated in the sentence. English does something similar with phrasal verbs:

• I ring up again
• I turn it off

So although ringe på belongs together in meaning, it does not always stay together visually.

Because Danish very often uses the present tense for future meaning when the context makes the time clear.

Here the sentence refers to a possible future situation:

• Hvis ingen åbner døren, ringer jeg på igen.

That is completely natural in Danish. English often does something similar:

• If nobody answers, I ring again is possible in some contexts, but English more often says:
• If nobody answers, I’ll ring again.

In Danish, using the present tense in both clauses is very common in this kind of real, likely condition.

Ingen means no one, nobody, or none depending on context.

In this sentence:

• ingen åbner døren = nobody opens the door

It is the most natural choice here.

A useful contrast:

• ingen = nobody / no one
• ikke nogen = not anyone / no one in some contexts, but often less direct here

For example:

• Ingen kom. = Nobody came.
• Jeg så ikke nogen. = I didn’t see anyone.

So as the subject of the clause, ingen is the standard, natural form.

Because døren means the door, while en dør means a door.

Danish usually forms the definite noun by adding the article to the end of the noun:

• dør = door
• en dør = a door
• døren = the door

So åbner døren means opens the door, referring to a specific door—most likely the one the speaker is standing at.

Here hvis is correct because this is a condition:

• Hvis ingen åbner døren ... = If nobody opens the door ...

Danish usually uses:

• hvis for if in conditional sentences
• om for whether/if in indirect questions

Compare:

• Hvis det regner, bliver vi hjemme. = If it rains, we stay home.
• Jeg ved ikke, om han kommer. = I don’t know whether he is coming.

So in your sentence, om would not be the normal choice.

Igen means again.

In this sentence:

• ringer jeg på igen = I ring again / I ring the doorbell again

Placing igen at the end is very natural Danish word order. It adds the idea of repetition after the main action has been stated.

So the flow is:

• action: ringer jeg på
• repetition: igen

That is the most neutral way to say it here.

Yes, the comma here is normal and expected.

The sentence begins with a subordinate clause:

• Hvis ingen åbner døren

Then comes the main clause:

• ringer jeg på igen

The comma marks the boundary between them:

• Hvis ingen åbner døren, ringer jeg på igen.

So the comma helps show where the if-clause ends and the main statement begins.

In meaning, yes, ingen is like English nobody/no one, so English uses opens.

But in Danish, this is less noticeable because Danish verbs do not change form according to person or number in the present tense.

So:

• jeg åbner
• du åbner
• han åbner
• ingen åbner

The form åbner stays the same. That makes Danish verb agreement much simpler than English.