Jeg åpnet bagasjerommet for å hente paraplyen, men der lå også en gammel pose med verktøy.

Bagasjerommet is the noun bagasjerom plus the definite article -et.

• bagasjerom = trunk / boot / luggage compartment
• -et = the for a singular neuter noun

So bagasjerommet means the trunk / the boot.

Norwegian usually attaches the definite article to the end of the noun instead of putting a separate word in front the way English does.

For å means to / in order to when expressing purpose.

So:

• for å hente paraplyen = to get/fetch the umbrella

Here it explains why the speaker opened the trunk. A very literal breakdown is:

• for = for
• å hente = to fetch / to get

In natural English, you would often just say to get the umbrella, but Norwegian commonly uses for å in this kind of purpose phrase.

Paraplyen is the definite form, meaning the umbrella.

• en paraply = an umbrella
• paraplyen = the umbrella

The speaker is referring to a specific umbrella they intended to pick up, not just any umbrella. That is why the definite form is used.

Here der means there or in there, referring to the trunk.

So:

• der lå også ... = there also lay ... / there was also ...

It is not det because det means it / that, not a location.

Compare:

• der = there
• det = it / that

In this sentence, the speaker is talking about where the bag was, so der is the right choice.

Norwegian often uses a position verb where English would simply use be.

• lå is the past tense of ligge = lie
• var is the past tense of være = be

With objects that are resting somewhere, Norwegian very often uses ligge/lå:

• Boken lå på bordet. = The book was lying on the table.
• Nøklene lå i veska. = The keys were in the bag.

So der lå også en gammel pose ... sounds more natural and more vivid than der var også .... Using var is not always impossible, but lå is the more idiomatic choice here.

This is because Norwegian main clauses usually follow the V2 rule: the finite verb normally comes in the second position.

Here the first element is der, so the verb lå has to come next:

• Der
  • lå
    • ...

Then også and the noun phrase follow.

This word order is especially common when introducing something in a place:

• Der sto en bil. = There was a car standing there.
• På bordet lå en bok. = On the table lay a book.

So der lå også en gammel pose is a very natural Norwegian structure.

Også means also / too / as well.

In this sentence, it means that besides the umbrella, there was another thing in the trunk: an old bag with tools.

Its placement is natural because Norwegian adverbs like også often come after the finite verb in main clauses:

• Der lå også en gammel pose ...

That placement gives the meaning there was also an old bag ...

This is adjective agreement.

Pose is a common-gender noun, so in the singular indefinite form you use:

• en gammel pose

Compare the patterns:

• common gender, singular, indefinite: en gammel bil
• neuter, singular, indefinite: et gammelt hus
• plural or definite: gamle

So:

• en gammel pose = correct
• et gammelt pose = wrong, because pose is not neuter
• en gamle pose = wrong, because gamle is not used for singular indefinite common gender

Because verktøy here is being used in a general sense: with tools.

Norwegian often leaves out an article after med when talking about contents, material, or something nonspecific.

So:

• en pose med verktøy = a bag with tools
• en pose med verktøyene = a bag with the tools

Also, verktøy is a word that often behaves like a collective noun in everyday use, similar to tools as a general category.

Åpnet is past tense:

• å åpne = to open
• åpnet = opened

But hente is in the infinitive because it comes after å:

• å hente = to get / to fetch

So the sentence combines:

• a main action in the past: Jeg åpnet ...
• a purpose infinitive: for å hente ...

That is why the verb forms are different.

Pose is a general word for a bag, often a soft or simple one.

Depending on context, it could mean:

• a plastic bag
• a shopping bag
• a small sack
• a loose bag or pouch

It usually does not suggest something structured like a suitcase. In this sentence, en gammel pose med verktøy sounds like some kind of old bag containing tools.

Because men joins two full main clauses here:

• Jeg åpnet bagasjerommet for å hente paraplyen
• der lå også en gammel pose med verktøy

In Norwegian, a comma is normally used before men when it links independent clauses like this.

So the comma helps show the break between:

• the action of opening the trunk
• the new piece of information about what was inside