Dette er en dobbel dør.

Dette means this.

In this sentence, Dette stands alone as a pronoun:

• Dette er ... = This is ...

By contrast, denne is normally used directly before a common-gender noun:

• denne døren = this door

So:

• Dette er en dobbel dør. = This is a double door.
• Denne døren er dobbel. = This door is double.

Because dør is not neuter.

In Norwegian, singular indefinite articles depend on gender:

• en for masculine, and also very often for feminine in Bokmål
• ei for feminine
• et for neuter

So you say:

• en dør
• not et dør

That is why the sentence has en dobbel dør.

Yes. Dør is a feminine noun, so in Bokmål both of these are possible:

• en dør
• ei dør

Both are correct in Bokmål. Many learners see en dør more often, because Bokmål often allows feminine nouns to use en. If you prefer to use feminine forms more actively, ei dobbel dør is also fine.

You may also see the definite forms:

• døren
• døra

Both are accepted in Bokmål.

Because adjectives change form to match the noun.

Here, dør is a singular non-neuter noun, so the adjective stays in its basic form:

• en dobbel dør

You would use dobbelt with a singular neuter noun:

• et dobbelt vindu = a double window

And you usually use doble with plural or definite forms:

• doble dører = double doors
• den doble døren = the double door

So in this sentence, dobbel is the correct form.

Often, yes: dobbeldør is very natural Norwegian.

Norwegian usually prefers compound nouns written as one word, so:

• dobbeldør is often the most idiomatic form for double door

That means many native speakers would probably say:

• Dette er en dobbeldør.

The version dobbel dør is still understandable and may appear in teaching material or as a descriptive phrase, but dobbeldør is often the more standard everyday spelling.

Yes, very similar.

The structure is:

• Dette = subject
• er = verb
• en dobbel dør = complement

So it works like English:

• This
  • is
    • a double door

This is a very straightforward Norwegian sentence.

The tricky part is the vowel ø.

A rough guide:

• dør sounds somewhat like durr or duhr, but with rounded lips
• the vowel is not the same as the English vowel in door

A useful approximation is:

1. Start with a vowel a bit like the one in bird for many English speakers
2. Round your lips more than you would in English
3. Add the r

The exact r sound depends on dialect, so do not worry too much about making it perfect immediately.

You would normally say:

• den doble døren
• or den doble døra

So the full sentence becomes:

• Dette er den doble døren.
• Dette er den doble døra.

Notice two changes:

1. You use den for the
2. The adjective changes from dobbel to doble

That adjective change happens because the noun phrase is now definite.

Yes, but it is slightly different in feel.

• Dette er en dobbel dør = This is a double door
• Det er en dobbel dør = It is a double door or sometimes a more neutral That is a double door, depending on context

If you are clearly pointing something out and mean this, then Dette er ... is the best match.

So the original sentence is a very natural choice when identifying something near you or in front of you.