Den vanntette posen holder klærne tørre.

Because pose is a common-gender noun in Norwegian.

• en pose = a bag
• posen = the bag

When a definite noun has an adjective before it, Norwegian usually uses:

• den for common gender
• det for neuter
• de for plural

So:

• den vanntette posen = the waterproof bag

If the noun were neuter, you would expect det instead.

This is a very common feature of Norwegian called double definiteness.

When a noun is definite and has an adjective before it, Norwegian normally marks definiteness twice:

1. with a separate word like den
2. with the definite ending on the noun, here -en

So:

• en vanntett pose = a waterproof bag
• posen = the bag
• den vanntette posen = the waterproof bag

This is normal Norwegian structure, even though it may feel redundant to an English speaker.

Because adjectives usually take the -e form when they come before a definite noun.

Compare:

• en vanntett pose = a waterproof bag
• den vanntette posen = the waterproof bag

So the adjective changes because the noun phrase is definite.

This -e form is very common in phrases like:

• den store bilen = the big car
• det lille huset = the small house
• de røde skoene = the red shoes

Vanntett is a compound adjective:

• vann = water
• tett = tight, closed, sealed

So vanntett literally means something like water-tight, which matches English waterproof.

This is a very typical Norwegian compound pattern.

Because klærne is the definite plural form: the clothes.

• klær = clothes
• klærne = the clothes

In this sentence, the meaning is not just clothes in general, but specific clothes being kept dry, so the definite form is used.

Because tørr has to agree with klærne, which is plural.

For this adjective, the basic agreement pattern is:

• tørr = singular common gender
• tørt = singular neuter
• tørre = plural

So:

• en tørr genser = a dry sweater
• et tørt håndkle = a dry towel
• tørre klær = dry clothes

In holder klærne tørre, tørre is describing klærne, so it must be plural.

Here holder means keeps, not literally holds in the hand.

The verb å holde has several meanings in Norwegian, just like to hold in English can have several meanings. In this sentence:

• holder klærne tørre = keeps the clothes dry

So the structure is:

• holde + object + adjective

Examples:

• Det holder maten varm. = It keeps the food warm.
• Jakken holder meg tørr. = The jacket keeps me dry.

It uses an object + object complement pattern.

Breakdown:

• Den vanntette posen = subject
• holder = verb
• klærne = object
• tørre = adjective describing the object

So tørre does not describe the bag. It describes the clothes.

This is similar to English:

• The bag keeps the clothes dry

where dry describes the clothes, not the bag.

Yes. Posen holder klærne tørre is perfectly natural and means The bag keeps the clothes dry.

The longer version Den vanntette posen holder klærne tørre adds extra information by specifying that it is the waterproof bag.

So:

• Posen holder klærne tørre. = The bag keeps the clothes dry.
• Den vanntette posen holder klærne tørre. = The waterproof bag keeps the clothes dry.

The sentence has standard Norwegian main-clause word order:

• Den vanntette posen = subject
• holder = verb
• klærne tørre = rest of the predicate

So it follows the common pattern:

• Subject + verb + object + complement

Norwegian main clauses usually place the finite verb in the second position, and that is exactly what happens here.

A few things may stand out for English speakers:

• vanntette: the double nn and tt show a short vowel before them
• klærne: æ is a front vowel that does not exist in the same way in English
• tørre: ø is another vowel English speakers often need practice with
• tørre also has a flipped or tapped r in many accents

You do not need perfect pronunciation right away, but the vowels in klærne and tørre are often the hardest part for English speakers.

The general idea is similar, but the exact forms are Norwegian.

For example, one very Norwegian-looking feature here is the double definiteness in den vanntette posen, which also exists in Swedish, while Danish handles some details differently in form and pronunciation.

So a learner should treat this sentence as specifically Norwegian, even if related languages may look similar.