Den gamle madrassen er så myk at lakenet faller av om natten.

Because Norwegian uses double definiteness in phrases like the old mattress.

Here is how it works:

• madrass = a mattress
• madrassen = the mattress
• gammel = old
• gamle madrassen = old mattress / the old mattress, but this is not enough by itself in standard Bokmål when a definite noun has an adjective before it
• den gamle madrassen = the old mattress

So when a noun is definite and also has an adjective, you usually need:

1. a definite article before the adjective: den
2. the adjective in its definite form: gamle
3. the noun in its definite form: madrassen

That is why den gamle madrassen is correct.

Because adjectives change form in Norwegian depending on the noun.

The basic pattern is:

• en gammel madrass = an old mattress
• et gammelt hus = an old house
• gamle madrasser = old mattresses
• den gamle madrassen = the old mattress

In this sentence, the noun phrase is definite: den gamle madrassen. In definite noun phrases, the adjective usually takes the -e form, so gammel becomes gamle.

Because madrass is a common-gender noun, and the definite singular form is made with -en.

• en madrass = a mattress
• madrassen = the mattress

So -en here is the definite ending, similar to English the, but attached to the noun.

This is a very common Norwegian structure:

• så ... at ... = so ... that ...

Examples:

• Hun er så trøtt at hun sovner med en gang. = She is so tired that she falls asleep immediately.
• Det er så kaldt at vannet fryser. = It is so cold that the water freezes.

So in your sentence:

• så myk = so soft
• at lakenet faller av om natten = that the sheet falls off at night

This is directly parallel to English so soft that...

Because after at, Norwegian usually uses normal subordinate-clause word order.

In the main clause:

• Den gamle madrassen er så myk ...

That is a normal main clause.

After at, you get a subordinate clause:

• ... at lakenet faller av om natten

In subordinate clauses, the word order is generally straightforward:

• subject + verb + other elements

So:

• lakenet = subject
• faller = verb
• av = particle
• om natten = time expression

This is normal Norwegian subordinate-clause structure.

Lakenet is the definite form of laken, which means sheet or bedsheet.

• et laken = a sheet
• lakenet = the sheet

It is a neuter noun, which is why the indefinite article is et.

Yes, in this sentence falle av works like a phrasal verb or verb + particle combination.

• falle = fall
• falle av = fall off

So:

• Lakenet faller av = The sheet falls off

This is very similar to English fall off.

The particle av often gives the idea of separation or coming loose:

• ta av = take off
• skru av = turn off
• rive av = tear off

Because lakenet is the subject, so it comes before the verb in this subordinate clause:

• at lakenet faller av

That literally follows the pattern:

• that + subject + verb + particle

You can sometimes see particles in slightly different positions in Norwegian, especially in main clauses, but lakenet faller av is the natural word order here.

Om natten means at night or during the night.

This is a standard time expression in Norwegian:

• om dagen = during the day / in the daytime
• om natten = at night / during the night
• om morgenen = in the morning

By contrast, i natten would usually sound unnatural here. I is often used for being inside something or for certain time expressions, but om natten is the idiomatic way to say at night in this kind of sentence.

This is just how several Norwegian time expressions are commonly formed. Norwegian often uses the definite form in fixed expressions of time:

• om morgenen
• om dagen
• om kvelden
• om natten

So even though English says at night without the, Norwegian naturally says om natten, with the definite noun form.

Because myk describes madrassen, and madrass is a common-gender noun, not a neuter noun.

Compare:

• en madrass er myk = a mattress is soft
• et teppe er mykt = a blanket/rug is soft

Adjectives used after verbs like er still agree with the noun in gender/number:

• common gender singular: myk
• neuter singular: mykt
• plural: myke

Since madrassen is common gender singular, myk is correct.

Both are in the present tense:

• er = is
• faller = falls

Norwegian present tense is usually formed by adding -r to the infinitive:

• å falle → faller
• å bo → bor
• å snakke → snakker

The verb å være is irregular:

• er = is / am / are

Not with exactly the same meaning.

• laken(et) = sheet
• sengetøy(et) = bedding / bed linen

So lakenet refers specifically to the sheet, while sengetøyet is more general. In this sentence, lakenet is more precise.

In Bokmål, madrass is a common-gender noun, which means it takes en in the indefinite singular:

• en madrass
• madrassen

It is better to think of Bokmål as having:

• common gender: usually en
• neuter: et

So yes, for practical learning purposes, you should learn madrass as an en-word.

Yes. The whole noun phrase den gamle madrassen is the subject.

Breakdown:

• Den gamle madrassen = subject
• er = verb
• så myk = complement
• at lakenet faller av om natten = subordinate clause explaining the result

Norwegian allows long noun phrases as subjects just like English does:

• The old mattress is so soft...
• Den gamle madrassen er så myk...

Yes. The part introduced by at expresses the result of the mattress being so soft.

Structure:

• cause/degree: så myk
• result: at lakenet faller av om natten

So this is a classic så ... at ... result construction:

• so ... that ...