Rusten er under døren.

Here, rusten is a noun meaning the rust.

That can be confusing, because rusten can also be an adjective meaning rusty. Compare:

• Rusten er under døren. = The rust is under the door.
• Døren er rusten. = The door is rusty.

So in your sentence, rusten is the subject of the sentence, not an adjective describing something.

Because Norwegian usually puts the definite article on the end of the noun.

So:

• rust = rust
• rusten = the rust

This is a very common pattern in Norwegian. English uses a separate word (the), but Norwegian often uses an ending instead.

For the same reason: døren means the door, while dør means door or a door, depending on context.

So:

• dør = door / a door
• døren = the door

In the sentence under døren, the meaning is under the door, so the definite form is used.

Yes. In Bokmål, both døren and døra can be used for the door.

• døren is a common Bokmål form
• døra is also very common, especially in everyday speech and in styles that use more feminine forms

So these are both possible:

• Rusten er under døren.
• Rusten er under døra.

They mean the same thing.

Er means is. It is the present tense of å være = to be.

So:

• jeg er = I am
• du er = you are
• han/hun er = he/she is

In this sentence, Rusten er under døren, it simply links the subject to its location: The rust is under the door.

Yes, very much so. Under is a preposition meaning under / below.

So:

• under bordet = under the table
• under senga = under the bed
• under døren = under the door

This is one of the easier prepositions for English speakers because it often matches English closely.

Yes, in this sentence it is very similar to English:

• Rusten = subject
• er = verb
• under døren = prepositional phrase showing location

So the structure is:

Subject + verb + place

That is why it feels familiar to an English speaker.

A useful extra point: Norwegian main clauses often require the verb to be in the second position, and this sentence already follows that rule naturally.

It is grammatical, but whether it feels natural depends on context.

Rusten er under døren sounds like you are talking about a specific, already known rust spot: the rust.

If you are just noticing rust for the first time, Norwegian might more naturally say:

• Det er rust under døren. = There is rust under the door.

So your sentence is fine, but it sounds more specific and definite than the more neutral Det er rust under døren.

The tricky part is ø.

A simple approximation:

• døren sounds roughly like DUR-en or DUH-ren, but with rounded lips

The vowel ø does not exist in standard English, so there is no perfect match. To make it:

1. Start with a sound somewhat like the vowel in bird or her
2. Round your lips more than you would in English

Also:

• døren has two syllables: dø-ren
• the r is clearly pronounced in many Norwegian accents, though it varies by region

Because Norwegian usually expresses the by adding an ending to the noun instead of using a separate word before it.

So instead of:

• the rust
• the door

Norwegian often has:

• rusten
• døren

This is one of the big differences between English and Norwegian noun grammar. English says the + noun, while Norwegian very often says noun + definite ending.